BALTIMORE, SEPT. 13 -- Triple Crown Productions announced revisions today in the nominating process for next year's Triple Crown races and in the point system that determines the million-dollar bonus.

Beginning in 1991, the Kentucky Derby, Preakness Stakes and Belmont Stakes will accept supplemental nominations, which allow a horse late access to the race at a much greater cost. A horse may be supplemented to all three races for $150,000, to the Preakness and Belmont for $100,000 and to the Belmont only for $50,000.

The deadline for early nominations (costing $600) remains Jan. 16. The late nomination deadline was extended three weeks to April 8, with a 50 percent raise in fee to $4,500. Any horse nominated after April 8 will be subject to the supplemental charges.

According to Ed Seigenfeld, executive director of Triple Crown Productions, the extension will allow horses to complete Santa Anita Derby weekend before being committed to the Triple Crown.

"We felt the extension of the late nomination deadline and allowing supplements give horsemen every possible chance to be in the Triple Crown races," Seigenfeld said.

The board of directors bowed to criticism of the 5-3-1 point system used to calculate the winner of the $1 million bonus for the horse who performs best in the three races. (A horse who wins all three nets $5 million, making the point system irrelevant.) The new formula gives 10 points to the winner of each of the three races, five for second, three for third and one for fourth.

Under this system, a horse can no longer win two legs of the Triple Crown and not at least share the $1 million bonus (assuming he or she runs in all three races, which is still a requirement). That fate befell Alysheba in 1987, the first year the bonus was offered.

Alysheba won the Derby and Preakness, but when he was fourth in the Belmont he lost the bonus, 11-10, to Bet Twice, who had finished second-second-first. With the amended formula, Alysheba would have won the bonus, 21-20.

"The revision of the point system reflects more accurately purse distribution and the traditional industry method of paying purses to fourth place," Seigenfeld said.

Whereas the Preakness and Belmont allowed supplemental entries for many years before the bonus was introduced, the Derby did not, denying many 3-year-olds -- particularly late developers -- an opportunity to compete. Among the most notable was Codex, who went on to win the 1980 Preakness and was favored in the Belmont.

Seigenfeld said he does not envision a "measurable change" in the purses of the three races, which are augmented by nominating and starting fees.

If any of the fields are oversubscribed, pre-entered horses will have priority over those supplemented. 'Sleek' Does It Again

Sleek N Graceful brought his unbeaten streak to three today, but it wasn't easy. Fortunate Lance clung to his left flank through the Pimlico stretch, but Sleek N Graceful refused to submit, winning by a head under Greg Hutton.

A 2-year-old son of Ack Ack, Sleek N Graceful ran the six furlongs on a fast track in 1:12 for the lowest win mutuel of the day, $3.60. Owner-trainer Larry Horning said the Bimelech Stakes might be next for his horse.