Lady Bird has continued to make public appearances
The 89-year-old widow of former President Lyndon Johnson is also "alert and in good spirits," according to Sandra Morgan, a spokeswoman for the Seton Medical Centre in Austin, Texas.
Another spokeswoman for the hospital said Mrs Johnson's daughter, Luci Baines Johnson, and other family members were at her side.
The exact nature of her illness has not been revealed yet, but her press assistant, Betty Tilson, told an Austin newspaper the former first lady was having problems speaking and the Secret Service thought it best to call an ambulance.
The newspaper also reported on its website that she had trouble swallowing.
George Christian, who was President Johnson's press secretary, said: "There's always a danger that it's serious, but at this point anyway she hasn't even been diagnosed."
Mrs Johnson was admitted to the same hospital in 1999 after fainting at her home, and also underwent cataract surgery that year. In 1993, she suffered what was described as a minor stroke.
Her husband, who was Democrat president from 1963 to 1969, died in 1973. They had been married for almost 39 years.
Lady Bird Johnson was christened Claudia Alta Taylor when she was born to a prosperous family near Karnack, Texas.
She got her nickname "Lady Bird" as a small child, but the name proved prophetic.
While in the White House, she became known as a determined environmentalist.
She met Lyndon Baines Johnson, then a Congressional secretary visiting Austin, in 1934.
Heart attack
He courted her from Washington with letters, telegrams, and telephone calls and seven weeks later he proposed to her.
In her own words: "Sometimes Lyndon simply takes your breath away."
She devoted herself to Johnson's political career, and after he suffered a heart attack in 1955, helped his staff keep things running smoothly.
He once said that voters "would happily have elected her over me".
The couple had two daughters, Lynda, in 1944 and Luci three years later.
Mrs Johnson has continued to make public appearances at the LBJ Library and Museum and at civic events in Austin.