The judge adjourned the case for the day after defendant Brian David Mitchell was removed on a stretcher and put in an ambulance.
Paramedics took Mitchell to a hospital, but neither court officials nor defense attorneys would identify it. His condition could not immediately be determined.
However, court officials said later in the day the trial would resume Wednesday.
The incident occurred as the jury was about to be called into the courtroom for another day of testimony.
Mitchell was sitting between two of his defense attorneys when he stopped his daily hymn singing and slumped over slightly in his chair. He cried out - a long, loud moan - as his body twisted to the left.
"Judge, I think he's having a seizure," defense attorney Wendy Lewis said, reaching for her client.
With the assistance of U.S. marshals who were standing guard in court, defense attorneys moved Mitchell to the floor.
Mitchell remained shackled as paramedics lifted his slight body onto a stretcher. Wearing an oxygen mask, he appeared pale and frightened. His eyes, which are usually closed in court, were wide and round, and both of his fists were clenched.
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Mitchell's former stepdaughter, Rebecca Woodridge, said the defendant has had seizures in the past and recently has been housed in a medical unit at the Salt Lake County Jail.
Woodridge said the first seizure she was made aware of was just after Christmas in 2009. She said Mitchell refuses all medical treatment because he doesn't like western medicine.
Jail spokesman Lt. Michael DeNiro said privacy rules kept him from commenting on medical issues of inmates.
Smart testified earlier in the trial that Mitchell suffered a seizure during the nine months she was his captive.
"He was in the middle of raping me and he experienced a seizure," Smart said in response to a question from the defense about whether she had seen Mitchell lose consciousness.
Jurors were not told Tuesday by U.S. District Judge Dale Kimball why the proceedings were suspended and only heard that unexpected issues had arisen.
"But we have every expectation that we can proceed again tomorrow," an apologetic Kimball said.
The trial is in its fourth week and is expected to last until Dec. 10.
Mitchell's attorneys don't dispute that Smart was abducted from her home at knifepoint and held captive, but they contend Mitchell is mentally ill and can't be held responsible for the crimes.
Mitchell has been disrupting court proceedings on a daily basis with hymn singing.
Smart was 14 when she was abducted from her home at knifepoint on June 5, 2002. She was recovered nine months later disguised in wig and sunglasses and walking a suburban Salt Lake City street with Mitchell.
Now 23, Smart has testified that she was forced to enter a polygamous marriage with Mitchell, endured near daily rapes, was forced to use drugs and alcohol, and was taken to California against her will.