A light rain and the threat of thunderstorms put a damper on the mood in the infield at Churchhill Downs today for the 103d running of the Kentucky Derby.

At Epsom, the famed race track in England, the cry on a rainy day is, "Put that bumper down."

In the infield here, the few persons carrying umbrellas in the crowd of 70,000 were told in simple, crude terms what they could do with their cloud covers.

"For $10 to get in here, we deserve better than to get drenched," complained Mark Keney of Louisville. "It's bad enough not being able to see the damn Derby."

One fan among the infield's 70,000 left early, and it wasn't because of the weather.

"He was knifed," said Dr. Leonard Kaplonski, an adviser with the Jefferson County Police Department's emergency medical squad. "He was 20 or 21, male, from out of state, and he was stabbed in the left flank with a very sharp instrument, in the area of the kidney. It's serious."

"We don't know who did it. No one's been apprenhended. We only know the knifing happened as the man was coming out of the tunnel enterig the infield," said Col. Russ McDaniel, the chief policeman on the beat.

"Most of the fights out here," McDaniel added, "are the end result of too much alcohol. Someone steps on someone else's blanket, or knocks over their food or drink. For the 100th Derby (in 1974), we had 100,000 people packed in; rail-to-rail people. That was bad, but 1967 was the worst.

"We arrested 500 kids. We don't know exactly what started it, except there was a lot of Big 10 and SEC football feeling in the middle of it. The end result was 11 of our troops in the hospital, one with a fractured law, one with broken ribs, another with a broken arm, and many with concussions."

Today, the "Woodstock-with-mutuel-windows" crowd was well-behaved, by comparison. Two volleyball games were in constant progress. About 100 Frisbees soared between the eighth poles. There was the usual quota of drinking, smoking, loving, even some occasional betting on the horses.

"They fight a little, pop a little somthing, smoke whatever's available, and try to stay alive," observed Bill Wetter, a policeman.

"The biggest problem is cut feet," said Lt. Leon Goss. "Your typical candidate for big trouble here, for our medical unit, is the guy walking around in his bare feet, drinking beer from an aluminum pull-tab can. Then he wonders why he winds up here with a bleeding toe or worse."

The Derby infield annually harbors a mass of Midwestern youth, most of whom are determined to get drunk or broke, or both, before the Derby is run. Michigan State had the largest contingent today, many East Lansing students following their band to this horse palace.

Tee-shirts outnumbered no shirts, 10 to 1. "Seattle Who?" one shirt asked. "I Love You, Seattle Slew," another replied.

Thd day's most popular jersey, however, was a white one with red lettering on the front, worn by many of the fillies. "You Wanna Horse Around?" it asked.

Occasionally, almost as an after-thought, the fans' attention focused on a horse race. The gates had opened at 8 o'clock. The first of 10 races left the gate at 11:30.

"I'm betting $5 to place, $5 to show on Sanhedrin," informed Suzie Walkman of Kankakee, Ill. "You're wasting your money," warned Tim Clarkson, her boy friend. "I'm going to go with Run Dusty Run, because Updike is my favorite writer."

The comment put Clarkson in line for the role of the infield's resident intellectual.

Many of the thousands of people brought six-packs and gallon jugs of apple cider.

Apple cider is a Derby tradition, as is bourbon and is sophisticated off-shoot, the mint julep - served for $2.50 a shot in a tall glass with ice, sugar syrup, bourbon and, of course, a sprig of mint.

On the other side of the main racing strip, the crowd was typical, except for their carrying more rain gear than usual. The swells were in the Skye Terrace, wonderfully protected from all the adverse (climate and otherwise) elements.

"There are more than 350 corporation and private planes out at Standiford Field," a Churchill executive proudly declared. "Some of them have been making two and three trips, back and forth, picking up guests."

The Terrace is home for this type of spectator, the Skye tables going as high as $2,400 for 16 guests. For this price, they were assured perfect insulation from all outside forces. Even the press and television get the cold shoulder.

"But they don't have half the fun we do, even if they can get drunk faster and easier," proclaimed Ted Rudman of Akron, back down in the infield incubater.

Rudman appeared to be surviving, an hour before the Derby, even though he'd begun the day by making a bad bet.

"I brought this plastic raincoat from a guy peddling them for $3 right outside the gates," he explained. "An hour ago, in here, the price was down to $2. Ten minutes ago, they were getting desperate: raincoats for $1. A smart guy would have passed them by completely, of course, but it figures: I'm a terrible handicapper."