This study tested the hypothesis that implicit power motivation moderates men's testosterone responses to victory or defeat in a contest situation. It also explored to what extent postvictory testosterone increases are associated with enhanced implicit learning of behavior instrumental for winning a contest. Salivary testosterone levels were assessed in 66 male adults several times before and after a contest whose outcome (winning or losing against a competitor on an implicit learning task) was varied experimentally. Among participants low in activity inhibition, a measure of impulse control, the power motive was a significant positive predictor of testosterone increases (15 min postcontest; r ؍ 0.71, P ؍ 0.01) and implicit learning (r ؍ 0.68, P < 0.05) after a victory, whereas it was a significant negative predictor of implicit learning (r ؍ ؊0.58, P ؍ 0.01) but not of testosterone increases (r ؍ ؊0.08, ns) after a defeat. Moreover, among participants low in activity inhibition testosterone increases were associated with enhanced implicit learning (r ؍ 0.38, P < 0.05) and there was statistical evidence that in winners testosterone increases mediated the effect of power motivation on implicit learning. Participants high in activity inhibition did not display this pattern of results. © 2002 Elsevier Science (USA) Key Words: testosterone; implicit power motive; motivation; activity inhibition; dominance contest; implicit learning.The steroid hormone testosterone has been implicated in social dominance and aggressive behavior in a wide variety of species, including humans (Bernstein, Gordon, and Rose, 1983;Mazur and Booth, 1998;Monaghan and Glickman, 1992). Specifically, many studies document rising testosterone levels in response to a dominance success but unchanged or even declining testosterone levels after a defeat (Bernstein et al., 1983;Mazur and Booth, 1998). Mazur (1985) has speculated that postvictory testosterone surges, which can be observed within minutes and up to some hours after a dominance success, may serve to reinforce behavior that was instrumental for achieving the victory. This hypothesis has gained plausibility by recent findings that document rewarding (e.g., Packard, Schroeder, and Alexander, 1998) and mood-enhancing (e.g., Rabkin, Wagner, and Rabkin, 1996) effects of testosterone administration.In humans, empirical support for a direct effect of winning and losing a competition on testosterone is equivocal. While some researchers report higher postcontest testosterone levels in winners than in losers (e.g., Gladue, Boechler, and McCaul, 1989;Mazur and Lamb, 1980;McCaul, Gladue, and Joppa, 1992), others have failed to obtain such a difference (e.g., GonzalezBono, Salvador, Serrano, and Ricarte, 1999;Mazur, Susman, and Edelbrock, 1997;Schultheiss, Campbell, and McClelland, 1999;Suay et al., 1999), thus raising doubts about the efficacy of situational outcomes to affect testosterone levels directly and reliably. Schultheiss et al. (1999) have therefore recently argued that motiva...