Teresa Heinz Kerry suffered a grand mal seizure, a medical source who treated her told NBC News.
That source called the seizure "an electrical storm in the brain."
The condition of Heinz Kerry, wife of U.S. Secretary of State John Kerry and heir to the Heinz ketchup company fortune, was upgraded from critical to fair Monday, a day after she was first hospitalized, a State Department spokesman said.
Heinz Kerry, 74, was flown to a Boston hospital Sunday after first being taken to by ambulance to a hospital on Nantucket, where the couple has a home.
"After conducting tests overnight and this morning, doctors at Massachusetts General Hospital have upgraded Mrs. Teresa Heinz Kerry's condition from critical to fair," Kerry spokesman Glen Johnson said in a statement.
Heinz Kerry is undergoing further evaluation, Johnson said. He added that her husband and other family members were with her at the hospital and the family was touched by the outpouring of well wishes.
Heinz Kerry was treated for breast cancer in late 2009 and has said she found out that she had cancer in her left breast after having an annual mammogram.
A month later, she underwent lumpectomies on both breasts after doctors also discovered what they thought was a benign growth on her right breast.
That diagnosis was initially confirmed in postoperative pathology, but two other doctors later found it to be malignant. She later had another pair of lumpectomies performed.
Heinz Kerry is the widow of former U.S. Sen. John Heinz and heir to the Heinz ketchup fortune. Heinz died in 1991 when a helicopter collided with a plane over a schoolyard in Merion, Pa. The senator was among seven people, including two children, who died in the crash.
Heinz Kerry and John Kerry married in 1995.
She enthusiastically participated in her husband's campaign for president in 2004 and became known for her strong opinions, sometimes attracting as much attention as the candidate.
Kerry accompanied his wife to the Boston hospital Sunday night and later went to his nearby home on Beacon Hill. He left the home shortly after 8 a.m. Monday and returned to the hospital by motorcade.
Before Sunday's emergency, John Kerry had been at the Nantucket home since returning from a nearly two-week, around-the-world diplomatic trip to the Mideast and Southeast Asia in the pre-dawn hours of July 3.
Before his wife's medical problem, he had planned to return Monday to Washington, where he and Treasury Secretary Jack Lew were to co-host high-level strategic and economic talks with senior Chinese officials on Wednesday and Thursday.
Kerry had also spoken of his desire to travel to Israel starting at the end of the week for what would be his sixth trip there as secretary. State Department officials said Kerry's schedule may now change depending on his wife's health.