"The Mississippi," a new CBS drama series premiering at 10 tonight on Channel 9, makes one long for a good "Perry Mason" rerun. There are strong similarities between the two shows but also distinct differences: Della Street never faced the prospect of rape by a sexual psychopath nor toddled off for nooners with fresh young attorneys.

Also, "Perry Mason" was a great television program. "The Mississippi" is more on the order of, Oh, Another Show About A Lawyer.

Ralph Waite, once Papa Walton, plays the lawyer, Ben Walker, who the credit montage tells us tired of the big city life and now operates out of a boat called "The Mississippi" on the river of the same name. Instead of a Della, he has a Stella, played by pretty Linda G. Miller, and instead of a Paul Drake, he has a Lafayette Tate, played by Stan Shaw.

As so often happened to good old Perrilla, as a few of his more intimate fans refer to him, Ben Walker's client tonight is charged with murder (of a snooty southern belle), and all roads lead to him. He owned the murder weapon, his fingerprints were all over it, and he was known to have left fingerprints on the deceased from time to time. Worse, he's considered the town troublemaker, so the odds are stacked against him.

Subpoenas are delivered, suspects interrogated, and somebody tries to crown Ben with a load of rocks. Values have of course changed since "Mason" days, but maybe not so much as the producers of this show think. The more we learn about the allegedly lovable defendant (Cotter Smith, who plays Robert F. Kennedy in the upcoming TV movie "Blood Feud"), the shakier his folk-hero credentials become. Oh sure, he smoked a joint in his time, says a friend, and then, oh sure, he dropped some acid, and oh sure, he slugged a cop and oh sure and so on. When the lawyer gets him off, and the kid says, "We sure stuck it to the establishment, didn't we?", one may be less than inclined to cheer.

The program is a Ralph Waite Production, no less, and Waite does have plenty of palatable smooth cool. He's a craggy Robert Young, though at times he looks a little like the late Arthur Godfrey. About the only improvements on "Perry Mason" are the racially mixed casting and the handsome location photography of Natchez, Miss. Jerry Ziegman wrote the premiere, Lee H. Katzin directed it, and, like much of prime time, it's easy to take, easier still to leave.