JOHANNESBURG, South Africa (AP) _ Winnie Mandela was in serious but stable condition Thursday night, suffering from a kidney ailment, her doctors said.

Mrs. Mandela enterted Johannesburg's Linksfield Park Clinic for tests on Tuesday, a day after ending her dispute with her estranged husband, President Nelson Mandela, by resigning from his Cabinet.

Initial reports indicated her condition was stress-related and not serious. However, the clinic's managing director, Dr. Peter Kalish, said Thursday that tests revealed Mrs. Mandela had ``a severe electrolyte imbalance affecting her kidney function, blood pressure and blood sugar.''

Kalish added in a statement that Mrs. Mandela's condition was ``in no way related to stress or any other possible psychosomatic disorder.''

She remained hospitalized Thursday.

Mrs. Mandela resigned from Cabinet on Monday, a day before her dismissal was to take effect.

Mandela first fired his wife as deputy minister of arts, culture, science and technology on March 27. Mrs. Mandela then went to court to challenge the dismissal. That prompted the president to re-hire her long enough to consult with other party leaders _ a technicality he had overlooked earlier _ then fire her again.

Mrs. Mandela is popular among grassroots members of Mandela's African National Congress, but the organization has strongly supported the president in the tug-of-war over her Cabinet post.

The Mandelas have been separated since 1992.