Champion mare Rachel Alexandra remains in a serious condition following surgery but is reported to be bright and alert.

The 2009 United States Horse of the Year was admitted to a veterinary hospital in Kentucky earlier this week for an emergency operation after giving birth to her second foal.

A statement from Stonestreet Farms said she underwent exploratory abdominal surgery to determine the cause of excess fluid in her abdominal cavity. A damaged section of her small colon was then repaired in a long and technically demanding procedure.

Owner Barbara Banke said veterinary surgeons reported her vital signs were normal on Friday afternoon (Saturday AEDT) and she was brighter and more alert.

"She looks better today," Banke said.

"Rachel is a determined fighter. It makes me feel better that she was happy to have a mint. I want to thank all her fans for their incredible devotion to Rachel. Your thoughts mean so much right now."

Her Bernadini foal continues to do well with a foster mare.