The rare excitement that only an undefeated, untested 3-year-old can engender promises to make Saturday's 103d Kentucky Derby at Churchill Downs a renewal to remember.

Majestic Prince was the last colt to begin the Triple Crown series with a perfect record, in 1969. He won. So did Morvich, in 1922, and the only filly to win. Regret, in 1915. No horse has ever completed the Derby, Preakness and Belmont still undefeated, however.

Now comes Seattle Slew, Six-for-six, a champion at 2, successful in the Flamingo and Wood Memorial at 3, with never a rival within 3 1/4 lengths of him at the wire.

"You look at this horse and you get the feeling he would destroy all these other horses in a fight . . . and I think they know it," a veteran New York horseman observed this morning. "Many of us believe he's that special."

The 110,000 to 12,000 fans forming the betting public undoubtedly will agree with that appraisal. They are expected to pound Slew down to 2 to 5 in the parimutuels. If the play on the favorite is heavier than that, which is possible, Karen and Mickey Taylor's dark bay son of Bold Reasoning will have set one modern Derby record before the 5:38 post time.

ABC will televise the 1 1/4-mile event that will gross $267,200 if the field of 15 remains intact. One or two colts might be scratched if, as predicted, overnight rain turns the racing strip to mud.

Only one Derby was richer, Cannonade's in 1974, with $274,000.

Seattle Slew has run less than six miles in competition - 46 furlongs, to be exact - and has always been in the lead. Critics contend he may lack the seasoning, the versatility and or the preparation necessary for America's most famous thoroughbred race.

"He's fit," 36-year-old trainer Billy Turner testified, willing to speculate that his colt "should be able to run the last quarter-mile in: 23-and-change if he has to."

Jean Crugnet, the occasionally controversial French jockey who is Slew's constant companion, believes there is only one horse in the race. His.

'He wins the Derby from here to across the street if I want,' Cruguet says. 'I have been trying to save him some in his race, though, and I'll try to save something for the Preakness and Belmont, if I can, in the Derby.'

Several rival trainers are not convinced Seattle Slew is that good. They want to see him subjected to pressure, early and late, before linking his name in the same sentence with that of Secretariat. Slaw in the Wood. No other horse has come as preference, is:

Run Dusty Run and Bob's Dusty (one entry) - 'Bob's Dusty was to have gone to the lead in the Blue Grass, when For The Moment won,' trainer Smiley Adams declared. Saturday, there should be no mistakes. Bob's Dusty will duel Seattle SLew early to benefit Run Dusty Run later.

For The Moment - 'His brother, Honest Pleasure second to Bold Forbes last year), wanted to run,' commented trainer LeRoy Jolley. 'This one was a brother in blood only until the Blue Grass."

Sanhedrin - Closed well to be second to Seattle Slew in the Wood. No other horse come as close to close to the favorite. 'We're hoping for fast early fractions,' says trainer Lou Rondinello of this confirmed stretch-runner.

Western Wind - Third in the Blue Grass, closing fairly well. Won three races in a row before that.

Steve's Friend - Owned by George Steinbrenner, who also has the New York Yankess in his sports stable. Won Hollywood Derby to earn trip to Downs. Lacks Mickey Rivers' natural speed.

Sir Sir, Papelote and Best Person (mutuel field - Papelote got a new owner yesterday, a man reportedly suffering from 'Derby fever.'

Affiliate - Trained by Laz Barrera, who saddled Bold Forbes, I was confident. With this horse, I'm hoping,' said Barrera of the Hollywood Derby runner-up.

Nostalgia - A top 2-year-old last fall. Came off pace to take Saturday's Stepping Stone mile in impressive style.

Giboulec - Second choice to Run Dusty Run in the Blue Grass but finished ninth in the slop. Wants fast track.

Flag Officer - Best-bred animal in the race, according to pedigree pundits. Breeding isn't everything.

Get The Axe - Keep showing signs of promise. Keeps disappointing.