Spain gave Aminta Zarate's great-great-grandfather 600,000 acres north of the Rio Grande River in 1792. Since then, Indian warfare, a war between the United States and Mexico and the establishment of the state of Texas have intervened, but Zarate will go to court Thursday in the District of Columbia in an effort to be compensated for land her family left in 1811.

She is among 2,000 plaintiffs in one of the longest-running disputes in the Southwest. It has been the subject of treaties, negotiations and hours of genealogical detective work.

And, like the Indian claims cases the dispute so strongly resembles, the litigation continues, a source of dreams of fantastic wealth for the dispossessed descendants, many of them impoverished.

Almost 200 years ago, 12 million acres south of the Nueces River and north of the Rio Grande were deeded to settlers by the governments of Spain and Mexico. Eventually, the original claimants and their families lost possession of the land--run off by Indians, run out of the United States after the U.S.-Mexican War, or denied access to it by new settlers in Texas in the late 19th century.

A claims commission attempted to settle the issue in the 1920s, but failed. At the time, the government of Mexico filed 433 claims for a total of 12 million acres against the United States, seeking either the return of the land or compensation for it. The land was valued at $193 million.

A 1941 treaty between the United States and Mexico resulted in an exchange of claims between the two countries, obligating Mexico to compensate the heirs--the heirs now claim--but again there was no immediate settlement.

Since then, the descendants of the original claimholders have sought a resolution of the issue with Mexico, but to no avail. Frustrated by the long and inconclusive negotiations, Zarate and others filed a class-action suit in September, 1981, in U.S. District Court in Washington, D.C., against the government of Mexico.

A hearing on the case is scheduled Thursday before U.S. District Court Judge Thomas F. Hogan.

Mexican authorities have hired the New York law firm of Millbank, Tweed, Hadley & McCloy to represent the country, and the lawyers have filed a motion to dismiss the suit on the grounds that U.S. courts do not have jurisdiction over Mexico in the matter.

Among other things, the case will test a 1976 act of Congress that gave greater access through U.S. courts to citizens who have disputes with foreign governments, and it could well end up in the Supreme Court.

"If we get jurisdiction, our attorneys believe Mexico will settle," Zarate said.

Descendants of the original settlers have been working on the case for decades. Aminta Zarate has been intimately involved only since 1977, but her role has been significant.

She helped form the Asociacion de Reclamantes in behalf of the Texas heirs, and has rummaged through tiny courthouses and ancient churches in Mexico and south Texas, seeking birth certificates, death certificates, wedding certificates and evidence of land titles in behalf of herself and other descendants of the original claimants. She estimates that the association has spent roughly $300,000 pressing for a settlement.

The association numbers about 2,000 people in the United States and Mexico, many of them poor. But there are perhaps 20,000 to 25,000 other descendants scattered throughout both countries who would be eligible to share in any settlement.

The value of the claims today would approach $1 billion, according to Robert Salazar, a Denver lawyer who, along with the Washington firm of Rogovin, Huge & Lenzner, is handling the suit for the plaintiffs.

Zarate would stand to benefit greatly from a settlement. Her great-great-grandfather, Jose Narciso Cavazos, received the largest single land grant of the 433 claims identified by Mexico in the 1920s. Cavazos died in 1803, and in 1811 his son was run off the land by Indians and never was able to reclaim it.

Mexico ceded Texas to the United States as part of the 1848 Treaty of Guadelupe Hidalgo that settled the U.S.-Mexican War. The treaty obligated the United States to honor the original claims to the south Texas acreage, but over the next 70 years the land was swallowed up by other Texas settlers.

Zarate said part of her great-great-grandfather's claim became part of the massive King Ranch, although that could not be immediately verified.

Much of the confusion over the parcels was a result of the U.S.-Mexican War. Many of the families of the original landholders fled Texas for Mexico after the war. Others simply saw their land fenced off at night. "A lot of land was taken over by carpetbaggers," said Gus Casas, the association's president.

Casas is an heir to three parcels of land totaling nearly 60,000 acres that were deeded to three sons of his mother's great-great-grandfather. "They were not really in a position to push for their land because of the uncertainty of their status after the war," he said.

In the 1920s, Mexico pressed the United States for a settlement of the claims, and in 1923 a claims commission was established jointly by the two countries. But it did not resolve a single claim presented by Mexico by the time its term ran out in 1937.

In November, 1941, the United States and Mexico signed a treaty settling the claims that had been filed with the commission.

The key part of the treaty, according to information supplied by the plaintiffs, was an exchange of claims. Thus, Mexico agreed to settle the 433 south Texas claims itself, and the United States agreed to settle claims it had made against Mexico. By 1948, the United States had upheld its part of the bargain.

The 1941 treaty became the basis for a series of negotiations between the Texas heirs and the government of Mexico. Representatives of the heirs say that Mexican officials repeatedly acknowledged their obligation to pay the claims and their intention to do so.

Neither officials of the Mexican government in Washington nor their attorneys would respond to inquiries this week about this and other aspects of the case.

"Mexico had decided to maintain the traditional Mexican practice of outwaiting and wearing down the claimants," an association publication says.

Special correspondent Anna Bennett contributed to this report.