Lexington is in the heart of the Kentucky horse country. And Saturday, just 65 miles west, in Louisville, sleek 3-year-olds will break from the starting gate at Churchill Downs for the 111th running of the Kentucky Derby -- America's most famous horse race. It will be the first event of racing's Triple Crown, which also includes the Preakness and the Belmont Stakes.

Horse racing is known as the Sport of Kings because long ago only the nobility took part, but it probably began in connection with warfare and chariot racing. Races apparently were held in some form at least as early as 1400 B.C. in Asia Minor; the first race for saddled horses was held in the Olympic games of 564 B.C. Today, there are more than 74,000 horse races a year in North America alone. But most true racing loyalists count seven of all races worldwide as among the most important -- at least in tradition and prestige. They are:

* Preakness Stakes, second jewel in the Crown, at Pimlico in Baltimore, on May 18.

* Epsom Derby, Britain's historic horse race. It has been running for 206 years and has spawned more than 200 similar classics around the world, including its Kentucky cousin. At Epsom, England, on June 5.

* Belmont Stakes, final race of the Triple Crown, at Belmont Park in Elmont, N.Y., on June 8.

* King George VI and the Queen Elizabeth Diamond Stakes, England's most important race for horses 3 years old and up, at Ascot Heath racecourse just outside London, on July 27.

* Jockey Club Gold Cup, traditionally the race that most often determines America's horse of the year, at Belmont on Oct. 5.

* Prix de l'Arc de Triomphe, the last big event winding up the European racing season, at Longchamp in Paris, on Oct. 6.

* Breeders' Cup, seven championship races -- with purses of a million dollars or more -- termed the Super Bowl of thoroughbred racing, at Aqueduct in Queens, N.Y., on Nov. 2.