Reggie Jackson went under the weather . . . and you didn't think the Yankees and their media would let the New Yorkers open defense of their world championship against Milwaukee today without turning up the cacophony?

Bronx Zoo is open for 1979: Reggie, they beseeched, when you were hospitalized first of the week in Florida, were you really being tested for something awful? Hepatitis? A Yankee coverup? . . . Reggie, the media reported, when he wasn't at practice in the stadium, won't be in uniform for opening day. Reggie, when he showed up in the clubhouse during the brief workout, said, yes he will-he feels "better," still "awful weak," but "I'll be available to pinch hit and I hope I can play Saturday."

Jackson said he "had diarrhea since March 20 and been feeling weak for a good while" before dropping out at midgame in Sunday's exhibition and being ordered by Al Rosen, club president, to the hospital for tests.

Rosen said, yes, medics mentioned a check for early stages of hepatitis, but the tests were negative, and he didn't want to spread scare stories until said tests were in.

A relieved Jackson: "The first time I took a test, it scared them and it scared the hell out of me."

Well, at least Ron Guidry won't have to be scared about Jackson's defense behind him in right field today against the Brewers . . . CAPTION: Picture, Tiger pitcher Dave Rozema shivers in Detroit; today's opener postponed. UPI