With change becoming the rule rather than the exception south of the border, why not a feminist first lady for South America's newest democratically elected president?

Why not, indeed, many of the 600 guests asked themselves last night at the Ecuadorian Embassy where president-elect Jaime Roldos, who takes office on Aug. 10, shared the spotlight with his wife, Martha.

Some guests were still talking about how earlier in the day at a press conference, Roldos had introduced his wife when a reporter asked about the make-up of his cabinet. Then Senora Roldos announced that for the first time in Ecuador's history, a woman will serve in the cabinet.

Later, when Roldos was asked again about the identity of his cabinet, the president-elect replied: "To know my cabinet you should ask my wife."

Last night, her response to that remark was that "Jaime is a very intelligent man and he, as president, will be the one who chooses. But the woman will possess the same traits of the other cabinet ministers - honesty, wisdom and capability."

"She's not only his wife, but his adviser and partner in politics," said Horacio Sevilla-Borja, Ecuador's charge d'affaires who, with his wife, hosted last night's reception.

While Roldos was closeted in a private office on a long-distance call to his vice president-elect in Ecuador, his wife discussed her role with reporters.

Had Rosalynn Carter been a role model?

"I consider the role Mrs. Carter is doing a role any woman would be proud of," she replied. "A wife shares joy and sadness, but a wife also collaborates with her husband in an effort to integrate his problems and to solve his problems, together. It is not a superficial role."

Founder of an Ecuadorian woman's organization which she thinks became a major factor in her husband's 68 percent popular vote, Senora Roldos said Ecuadorians were surprised at the active role she played as a campaign adviser.

"But now people welcome it and understand."

Sen. Edward Zorinsky (D-Neb.), chairman of the subcommittee on affairs of the western hemisphere, said he did not know if Senora Roldos would accompany her husband to a breakfast meeting Senate colleagues are giving this morning.

"Wives don't ordinarily attend, but she is certainly welcome."

Surprisingly, there was little sustained talk about the fall of the Somoza government. Undersecretary of State David D. Newsom, ranking State Department official, kept his own counsel, disappearing into the crowd of guests from Latin American and OAS embassies, the World Bank, the International Monetary Fund and the Ecuadorian community.

Miami publisher Francisco Aguirre said whoever the next Nicaraguan ambassador is, "He'll be a simple guy - and skinny."

Embassy rooms had been newly redecorated for the occasion and were bare of furnishings. Social Secretary Cary Casares quickly corrected that yesterday. She not only brought her own art collection from home, but borrowed those of her friends.

"Everybody helped," she said. CAPTION: Picture, From left, Martha Roldos, president-elect Jaime Roldos, Horacio Sevilla-Borja and Dessima Williams; by Joe Heiberger - The Washington Post