Subadult male prairie voles were implanted with testosterone in Silastic tubing in an attempt to manipulate aggressive behavior. The implanted voles exhibited a higher level of aggression than either controls or shams in pairwise encounters in a neutral arena. Release rates of testosterone from Silastic tubing were measured in castrated males by radioimmunoassay. Serum testosterone titer of each group of castrated males implanted with three different sized capsules rose to a maximum on the 4th day after implantation and declined to a minimum thereafter. Group mean titers declined in the order of their capsule sizes. It was concluded that implants of testosterone in Silastic tubing are an effective method of increasing aggressiveness in male prairie voles. The possible mode of action of testosterone mediated aggression is discussed.