Boxer was asked why at a Senate hearing last summer she insisted Brig. Gen. Michael Walsh, of the U.S. Army Corps of Engineers, call her "senator" and not "ma'am."

"In that particular moment, we were having a lot of back-and-forth," Boxer said. "This was a formal hearing, so I made the call that I should call the general 'general,' and it would be better if he called me by my formal title, instead of 'sir,' 'ma'am,' 'general,' 'senator.' "

Boxer said she offered to apologize to Walsh.

"Afterwards, I called the general and I said, 'Do I owe you an apology? Did I upset you?' He said, 'No, not at all.' And we work very well together," Boxer said.

Fiorina said she was "pleased" to hear Boxer had called the general to offer an apology.

The debate was the first between the two women and included questions on issues such as climate change, gun control, stem cell research and abortion.