This laboratory study assessed the effectiveness of warning messages intended to aid in the control of gambling. Participants were 120 undergraduate students from an urban state university who reported previous gambling activity. They were recruited to play a computerized roulette game with imaginary money. Money left at the end of play was exchanged for raffle tickets for a prize drawing. Participants were randomly assigned to 1 of 2 conditions. In the warning-message condition, participants received an educational component discussing common irrational beliefs expressed by gamblers and, while playing roulette, viewed brief messages that addressed irrational gambling beliefs. In the control condition, participants received an educational component on the history of roulette but no warning messages. Participants in the warning-message condition reported significantly fewer irrational beliefs and demonstrated less risky gambling behavior than those in the control condition.