In 1984, a number of US cyclists used blood transfusions to boost their performance at the Los Angeles Olympic Games. The cyclists broke no rules and dominated the Games, yet were later maligned as cheaters and dopers?they had, it seemed, violated some important norm, albeit one which was neither an official rule nor otherwise easily identifiable. Their case illustrates the moral ambiguity that arises when a performance enhancement is employed in a sport that has not addressed it. This article takes (...) up the crucial question posed by such moral ambiguity: is it ethical to enhance performance through a substance or technology when no rules exist to prohibit it? We first examine ordinary ethical obligations that athletes carry based on their status as moral agents. We conclude that such obligations provide some guidance, but cannot resolve the issue. We then examine arguments that take sport as a unique social practice that presents its own moral obligation not to use performance enhancers. We argue that these ?spirit of sport? arguments, developed by McNamee and Loland and Hoppeler cannot resolve the issue either. We conclude that when the rules governing sport are silent on the issue of performance enhancement the constraints on its ethical use are limited at best. (shrink)