The frail 94-year-old was discharged from the Ronald Reagan UCLA Medical Center on Monday after receiving treatment for severe pneumonia-related symptoms, but she was back in doctors' care 24 hours later.
Her husband, Prince Frederic Von Anhalt, tells TMZ.com the veteran star underwent an operation to treat a "major infection in her stomach", which was caused by a feeding tube.
She lost consciousness in the early hours of Wednesday and has been unresponsive since.
Gabor has been in and out of hospital since she broke her hip in a fall last summer. In January, part of her leg was amputated to stop the spread of gangrene and fears for her health mounted in March when she began coughing up blood.