Former Health and Human Services Secretary Kathleen Sebelius is taking the witness stand Tuesday in Sen. Robert Menendez's federal corruption trial.
She is appearing under subpoena, the Justice Department said.
Prosecutors say the New Jersey Democrat personally pressured Sebelius to intervene in a multimillion-dollar Medicare dispute involving the senator's friend and co-defendant, Dr. Salomon Melgen.
Melgen, a wealthy ophthalmologist from Florida, was hit with a demand from the Centers for Medicare and Medicaid Services in 2009 that he repay $8.9 million in alleged overbillings related to his use of the eye drug, Lucentis.
Prosecutors say Menendez then pushed high-level federal officials, including Sebelius and then-Senate Majority Leader Harry Reid, D-Nevada, to help Melgen favorably resolve the dispute in exchange for free rides on Melgen's private jets, political donations and other perks.
The men maintain they broke no laws.
A former Centers for Medicare and Medicaid official on Monday described to the jury what he recalled as an "angry" and "hostile" tone from Menendez during a meeting with Sebelius and Reid in 2012.
"I found the tone to be very angry, very hostile," Jonathan Blum said. "I found that I was being put on the defensive and it was a very angry exchange."
Blum said that as the meeting ended, Menendez told Sebelius he would continue to "press the issue going forward" through his role on the Senate finance committee. That committee shared oversight responsibility over HHS.