Four outstanding American horses, all breeds developed in the United States, are appearing on a block of four commemoratives being put out this Wednesday in Lexington, Ky.

The first-day ceremony for the new issue, which depicts the quarter horse, the Morgan, the saddlebred and the appaloosa, will be at the state-operated Horse Park, which features more than 30 breeds of horses, mules and donkeys.

The stamps follow up last year's block of pedigreed dogs. Both blocks were designed by Roy Andersen, a Sedona, Ariz., artist who depicted the horses in oil, using rich earth tones. The four different stamps in the block are se-tenant, a philatelic term for holding one another.

A statuesque quarter horse and another grazing in the distance appear on the stamp at the upper left. That horse is the oldest of American breeds and widely regarded as the most popular and most versatile horse in the world.

The quarter horse depicted is chestnut, with a characteristic short head and a well-developed neck on a compact body, a muscular horse with elegant legs.

The breed has a remarkable instinct for working with cattle. It traveled west with the pioneers and was indispensable in the great days of cattle herding.

At the upper right of the block is the Morgan, with a church steeple in the background to suggest its Vermont habitat. The Morgan Horse Farm of the University of Vermont is now a permanent memorial to the breed. A small horse, but powerful and versatile, it is shown with characteristic short head, deep chest and full mane and tail.

Few breeds of horse can claim descent from one common sire as the Morgan can. That stallion, in the early 1800s, was a sire astonishing in what geneticists call prepotency, the ability of one parent to impress its hereditary characteristics on its progeny. Foals sired of dams of a variety of breeds bore their sire's physical traits so often that a new breed began.

A striding saddlebred, known initially as the Kentucky saddler because it was largely in that state the breed evolved, appears on the lower left of the block. The horse, in many different colors and with white markings, has a small head, an arched neck and an exaggeratedly high tail, which is often artificially "set" for a higher carriage.

At the lower right is an appaloosa, compact and large boned, with sparse mane and tail. The breed is distinctive for its coat patterns -- variations of white over loins and hips, with or without spots, mottled over a white body or snowflaked. The horse was meticulously bred by the Nez Perce' Indians of the Northwest, who lived along the Palouse River. Their unusual horses became known as Palouse horses, which became slurred into appaloosa.

In 1877 the Nez Perce' under Chief Joseph were crushed by the U.S. Cavalry and almost all their horses were slaughtered. Today the breed has made a comeback and is among the nation's biggest breeds. It is a favorite as a circus horse and for Wild West shows.

The semi-jumbo size of 40 stamps per post office pane has been used for the horses, which appear in five colors: yellow, magenta, cyan, black and black tone. There is one five-digit plate number per pane.

Collectors of first-day-of-issue cancellations have a deadline of Oct. 25 for sending in orders and the usual alternatives in ordering.

Collectors affixing stamps on their envelopes, which must bear addresses, should send first-day covers to Customer-Affixed Stamps, American Horses Stamps, Postmaster, Lexington, Ky. 40511-9991.

Collectors preferring full processing by the Postal Service should send their addressed envelopes to American Horses Stamps, Postmaster, Lexington, Ky. 40511-9992. The cost is 22 cents per stamp affixed on a cover. Personal checks are accepted, cash is not welcomed, payment by stamps is rejected.