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Ex-MTV "Road Rules" star Brian Lancaster has died at just 43 years old ... TMZ has learned.

Brian appeared on season 7 of "Road Rules" back in 1999. His family tells us he was found dead in his Pennsylvania home last Thursday. They believe it was heart failure, and tell us he had a history of arrhythmia.

The family says the death was not drug or alcohol related -- and, as far as they know, no substances were found in the home.

Brian was 23 years old when he competed on "Road Rules." He didn't win the grand prize of $43k, but he competed on 13 different "missions" over the course of 15 episodes while shooting in Mexico, Costa Rica and the United States.