In a few days, one of television's ratings of passage begins anew with the May sweeps period.

This is a thrice-yearly custom during which the networks fatten up their schedules with expensive mini-series and movies, and local television stations suddenly discover the news value of sex.

The purpose of all this is to boost the ratings that establish advertising rates.

So here's a brief look at what to expect over the tube in the coming weeks. In some cases, exact times and dates were not available, so be sure to check TV Prevue for programs you don't want to miss.

ABC (WLS-Channel 7)

ABC kicks off the month of May with a "Disney Sunday Movie" titled "Splash Too," which stars Amy Yasbeck and Todd Waring in a sequel to the box-office hit "Splash."

Also airing May 1 is the first installment of the two-part mini-series, "Onassis: The Richest Man In The World." It stars Raul Julia as Aristotle Onassis, Jane Seymour as Maria Callas, Francesca Annis as Jackie Kennedy Onassis and Anthony Quinn (who once played Ari himself) as Socrates Onassis. The mini-series concludes the next night.

On May 8 and 9, ABC weighs in with another mini-series, Robert Ludlum's "The Bourne Identity," starring Richard Chamberlain in a rare non-period piece and Jaclyn Smith.

George Carlin stars in a two-part "Disney Sunday Movie" titled "Justin Case," on May 15 and May 22.

And baseball fans will be happy to know this year's "ABC Monday Night Baseball" swings into action on June 6.

CBS (WBBM-Channel 2)

CBS has several specials on tap for sometime in May, including "Internal Affairs," starring Richard Crenna in a reprise of his role as detective Frank Janek from last year's successful CBS movie "Double Take." Kate Capshaw and Cliff Gorman co-star.

Also on tap for an unspecified date in May is "The Beryl Markham Story," starring Stephanie Powers as the first woman to fly east-to-west across the Atlantic Ocean.

Director Robert Altman, who has been doing some terrific work in television lately, brings forth an adaptation of the successful stage play "The Caine Mutiny Court Martial," starring Eric Bogosian, Jeff Daniels, Brad Davis, Michael Murphy and Peter Gallagher.

CBS also will be televising the Miss Universe pageant on May 23 and theater's Tony Awards on June 5.

NBC (WMAQ-Channel 5)

Like CBS, NBC also has several special events for sometime in May.

"Invader" is the story of a female alien life form who comes to earth to help a "street-smart cop." Maryam D'Abo and Joe Cortese star.

Lindsay Wagner will star in "The Taking of (TWA) Flight 847," about the hijacking and 17-day hostage situation in 1985, while Eric Roberts will star in "To Heal A Nation," the story of the creation of the Vietnam War veterans memorial in Washington.

Raymond Burr will return in the eighth "Perry Mason" movie. And "The Incredible Hulk Returns," with Bill Bixby and Lou Ferrigno.

PBS (WTTW-Channel 11)

A varied tribute to J.S. Bach, "All That Bach," will be presented on May 27. Jazz, classical and dance interpretations of the great composer highlight the schedule.

On May 16, PBS will air "An Ocean Apart," a study of relations between Britain and the United States.

"Richard Tucker: The American Caruso," airing May 13, will honor the career of the great American tenor.

Veteran newsman Paul Duke will host a two-part series, "This Honorable Court," examining the workings of the U.S. Supreme Court on May 2.

The critically acclaimed (at least by me) series "American Playhouse" will feature Jack Lemmon in Eugene O'Neill's "Long Day's Journey Into Night" on May 4, as well as the potentially controversial "The Trial of Bernhard Goetz," starring Peter Crobie in the title role, on May 11.

"Frontline," the superb documentary series, will examine racism on May 10, CIA involvement with drug dealers May 17 and the NATO alliance May 24.

Local WTTW productions will include "The Golden Apple Awards for Excellence In Teaching" on May 20, "Dolphins of the Seven Seas," on May 25 and the "Illinois Young Performers Competition" on May 31.

Fox Broadcasting (WFLD-Channel 32)

Fox Broadcasting will introduce one series, "The Dirty Dozen," based on the popular film and starring Ben Murphy, on April 30.

Cable stations

The pay service HBO will present a four-hour live tribute to rock 'n' roll with "The Atlantic Records 40th Anniversary Tribute: It's Only Rock 'n' Roll" on May 14, featuring Mick Jagger, Keith Richards, the Who, The Rascals and possible reunions of Led Zeppelin and the Eagles. Also on HBO, "Women of the Night II," on May 21, will be an evening of female stand-up comics, while the movie "Clinton and Nadine," starring Ellen Barkin and Andy Garcia in a gun-running drama, will be presented May 28.

The pay service Showtime has tentatively scheduled for May "Gotham," a romantic thriller starring Tommy Lee Jones and Virginia Madsen. Also the situation comedy "Brothers" will begin its fourth season and Fleetwood Mac will be featured in concert on May 21.

The Disney Channel, another cable pay service, will present the children's classic "Lady and the Tramp" on May 7. It also plans to present "Extraordinary Summer Olympic Athletes" on May 20. And there is a Henry Fonda film festival throughout the month.

Furthermore, Disney will carry live "The Prairie Home Companion's Garrison Keillor's Second Farewell," from Radio City Music Hall on June 4, The show is part of a Disney Channel free preview weekend (June 4-5).

Over on the basic cable channel Arts and Entertainment, the British comedy series "Alas Smith and Jones" begins another season on May 1. And another installment of "A&E's Live at the Improv," featuring some of the country's top comedians, will begin May 6.

Daniel Ruth is television critic of the Chicago Sun-Times.