The defense rested its case.

Stinging from criticism for committing 50 errors in the first 34 games, the Royals finished an errorless week at home today with several stunning defensive plays in a 4-0 victory over the Orioles.

The Royals got to Dennis Martinez for 10 hits, handing him a league-leading ninth loss. One hit, a first-inning home run by rookie right fielder Pat Sheridan, was enough to beat him this day.

"He gave that kid what you call a blue-darter home run," Orioles Manager Joe Altobelli said. "That was some blast."

Sheridan's home run rose high with a 20-mph wind to right-center, soared above the 385 marker, cleared an advertising billboard and splashed down in the upper tier of the bilevel waterfall spectacular that decorates the outer regions of Royals Stadium.

Said Sheridan, a skinny 6-foot-3, 175-pounder whose two long home runs have tied and won games: "I've never been known as a Judy."

His punch was enough because of the defensive backing provided for veteran left-hander Paul Splittorff, including a first-inning play he helped himself with.

The first two Orioles singled in the first, John Shelby and Dan Ford. Cal Ripken then bounced to the mound and Splittorff turned a double play with second baseman Frank White and first baseman Willie Aikens.

Three times in the first five innings the Royals made double plays, and four times the Orioles had two runners on base and didn't move them.

That included a pair of two-out singles off Dan Quisenberry in the ninth before he finished his 11th save, tops in the league along with Bill Caudill of Seattle.

Among the several sparklers afield for the Royals--shortstop U.L. Washington made one, third baseman George Brett started a double play with another--White's hurt the Orioles most.

"White saved them two or three runs himself. He was all over the place," said Gary Roenicke, who twice had almost-certain hits stolen by White.

One was on a grounder up the middle leading off the second inning. White shagged it behind second base and threw off balance in time.

That was merely the preview. The main feature came in the third after Leo Hernandez led off with a single.

Rick Dempsey followed with a scorcher up the middle that appeared headed for center field. White stopped it with a head-long dive, flipped backhanded to Washington and started a double play that surely left the Orioles blinking in disbelief.

"He's like having two right fielders out there," Roenicke said.

After that play the Royals expanded on their 1-0 lead piecemeal. It went to 2-0 in the third on a pair of walks, Willie Wilson's 20th stolen base and a single by Hal McRae.

George Brett singled in the fifth with two out and scored when McRae's routine fly to center popped out of Shelby's glove. Shelby, who made an exceptional catch and throw that helped preserve the Orioles' 1-0 victory the night before, had a dreadful day with the wind. He misplayed a fly by Aikens in the fourth into a triple, and when he caught a fly by White in the eighth the crowd gave him an ovation.

The final run scored on Washington's opposite-field double that followed John Wathan's single and stolen base, the seventh steal of the series by the Royals in nine attempts.

While the Royals were evening the four-game series their premier right-hander, Dennis Leonard, was undergoing knee surgery that probably will keep him from pitching again this year.

Doctors said long-range prognosis is "excellent" on the knee that buckled as Leonard pitched to Ripken on Saturday night. Leonard will be in a cast six to eight weeks, then be subjected to about a month of bending exercises before being allowed to throw again.