Undefeated Seattle Slew will attempt to sweep the Triple Crown here Saturday when he matches strides with nine rivals in the 1 1/2 mile Belmont Stakes, which is often referred to as, "The Test of Champions."

A dark bay colt, Seattle Slew has been perfectly spotted by trainer William Turner Jr., who has done what other, more prominent horsemen usually give only lip service-give his horse a chance to develop.

As a 2-year-old, Seattle Slew showed he was exceptional. After winning two sprints at Belmont Park, Turner, who apparently has always known what he had in Seattle Slew, skipped other conditioning races for which his horse was eligible and decided to enter the son of Bold Reasoning in the Champagne Stakes at a mile.

Seattle Slew justified Turner's confidence by winning the Champagne in an incredible 1:34 2/5. No 2-year-old since the mighty Count Fleet had run that fast, And it was here that Turner and the owners of tomorrows prohibitive choice made their wisest move. They rested Seattle Slew for five months and let him develop. Most only talk about doing something like that; Slew's connections did it.

Seattle Slew has started five times as a 3-year-old. Incredible as it may seem, it was not until his race in the Preakness that he duplicated in his performance in the Champagne as a 2-year-old. Turner, by design, has run his horse less than at peak of form. His stated aim was to have Slew at his peak for the Belmont, then give him a rest until the fall.

Seattle Slew, it says here, will not be at his peak Saturday. Despite his improved performance in the Preakness, Slew has not done enough to bring him up to his absolute best form.

A race horse-or a human athlete, for that matter-to reach his peak there must be a conditioning period in which the muscles and circulatory system are prepared for hard work. Then, in stages, the horse or human must tire and even exhaust the body, then be rested. At each plateau, the capacity of the muscles, heart and lungs is increased.

For the Belmont Seattle Slew has, at best, been kept at his Preakness level, which is only slghtly better than his best race as a 2-year-old. His mile move in 1:38 2/5 in th fog, and his three quarters in 1:11 and change since the Preakness are not enough to put him at his peak. It is significant that Slew ran his best race as a 3-year-old in the Preakness after having raced hard in the Derby.

The emphasis here has been on Seattle Slew's condition because it is the central issue in the race. Had Turner, say, been content to let Slew just gallop into the Belmont after his recent mile work, the race would become interesting, indeed.

Horses like Sanhedrin, Run Dusty Run, Iron Constitution, Spirit Level and Mr. Red Wing all figure to run the Belmont distance in 2:29 on a fast track. That's about four lengths too slow for Slew, presuming he runs his second-best race. Should he ever run to his 3-year-old potential, the mighty Forego will have to eat his dust, too.