Bonds' mistress of nine years, Kimberley Bell, spent most of the day in the stand in exhausting sparring with Bonds' defense lawyer Chris Arguedas, who at times seemed barely able to contain her disdain for Bell.
Bell testified that in 1999 she asked Bonds what caused an elbow injury. She told the jury he said, "It was because of steroids."
"This is the first time we've heard direct testimony about Barry's state of mind," said ABC7 legal analyst Dean Johnson. "He knew he was using steroids."
Bell testified to physical and emotional changes she observed in Bonds starting in 1998 -- back acne, dramatic overall body size increase, bloating, hair loss, shrunken genitals, sexual dysfunction and increased aggression. Those are all symptoms of anabolic steroids and human growth hormone or HGH.
She testified he threatened her. "He said he'd cut off my head and leave me in a ditch."
"The government wants, I think, the jurors to put all of that together and come to the conclusion there's no way Bonds could have taken these substances and not known what he was doing," said "Game of Shadows" author Mark Fainaru-Wada.
Bonds is on trial for telling a 2003 grand jury that he never knowingly took anabolic steroids and HGH.
On cross-examination, Arguedas tried to paint Bell as an angry, vindictive, scorned former girlfriend out to humiliate Bonds and capitalize on his fame with a book about their relationship. It was never published.
Bell was expecting to be paid $100,000 for posing in Playboy, but testified in the end she got only about $18,000.
At one point, the judge told Arguedas to ratchet down her argumentative tone.
"It's very obvious that the Bonds defense team has made a conscious, strategic decision that they're going to be as aggressive as possible with this key witness," said Johnson. "Whether that strategy works out, only the verdict will tell."
Bell testified at one point that she was afraid Bonds might kill her. She told the jury that Monday was the first time she had seen him since he told her over the phone in 2003 to "disappear."