A rebellious Maryland Senate rejected the recommendations of its leadership yesterday and voted to weaken significantly the powers of a proposed stadium authority by requiring legislative approval of any site for a new sports stadium.

The requirement, approved 25 to 22, and other Senate changes in the legislation would render the proposed Maryland Stadium Authority ineffective, said Senate President Melvin A. Steinberg (D-Baltimore County).

Making the selection of a stadium site contingent on the General Assembly's approval would create numerous political delays in any attempt to build a replacement for Baltimore's Memorial Stadium, proponents of a strong authority said.

While agreeing amendments passed by the Senate would cripple the proposed authority, Sen. Dennis F. Rasmussen (D-Baltimore), the Finance Committee chairman, held out hope that some type of compromise could be reached before the end of the session Monday.

"This bill has got a long way to go. A lot of things could happen before this bill comes back up for a vote," he said after failing to block the amendments on the Senate floor.

"I think this right now is all kind of immaterial," Rasmussen said.

The House has already approved its own version of the stadium legislation. The Senate must vote one more time on its bill. A conference committee is expected to be appointed to work out differences between the House and Senate measures.

The Senate's bill would restrict construction of any new stadium to the Baltimore metropolitan area of Baltimore City and Anne Arundel, Baltimore, Carroll, Harford and Howard counties. It also would require the authority to include representatives from all sections of the state. 'Pain and Suffering' Cap

The Maryland House of Delegates yesterday gave tentative approval to a $425,000 cap on "pain and suffering" awards in personal injury cases but not before nearly an hour of debate from lawmakers concerned about people who might be hurt by the legislation.

Del. Gary R. Alexander (D-Prince George's) said that crime victims and those injured by defective automobiles or tainted drugs could be hurt by the limits.

"Why are we doing this to help car manufacturers?" Alexander asked. "Why are we doing this to help drug manufacturers? Why are we broadening this to include every kind of personal injury case?"

Other legislators expressed concern that the measure, which is aimed at holding down insurance costs, also could limit damages awarded to victims of airplane crashes and other catastrophes.

The House Judiciary Committee decided to broaden the bill to include all personal injury cases because the problem is universal, said Committee Chairman Joseph E. Owens (D-Montgomery).

"You got to realize some people will be victims" under the cap, said Owens. "But in 99 percent of the cases, the cap would cover it."

Delegates also questioned whether adopting the cap on noneconomic awards would ensure that insurance would be available and premiums would come down.

"My concern is come May 1 or June 1, that we are told [by the insurance companies] that we didn't do enough," said Del. Larry Young (D-Baltimore).

To become law, the bill must receive final approval by the House and then the two chambers must resolve the differences between the two versions. The Senate bill limits noneconomic awards in medical malpractice cases to $350,000. New Adviser Joins Hughes

Gov. Harry Hughes, apparently bent on mending fences with voters as he prepares a run for the U.S. Senate, hired a seasoned political adviser yesterday to serve as his liaison with community groups around the state.

Hughes hired Dan D. (Zack) Zaccagnini as a new executive assistant.

Zaccagnini, who will be paid at an annual rate of $51,800, has worked for a virtual Who's Who of Maryland politicians, serving as special assistant to former Baltimore mayor Thomas D'Alesandro III, U.S. Sen. Charles McC. Mathias Jr. (R) and Rep. Helen D. Bentley (R).

Hughes said his new assistant would bring "experience in the federal arena" to the marble halls of the State House in Annapolis.