A wealth of evidence has linked individual differences in testosterone (T) to social, cognitive, and behavioral processes related to human dominance. Moreover, recent evidence indicates that a single administration of T reduces interpersonal trust in healthy young women. Here, in a sample of men and women (n=96), we investigated the extent to which endogenous fluctuations in T during a competitive interaction would predict subsequent ratings of trust from emotionally neutral faces. Results indicated that a rise in T predicted a decrease in trust ratings in men, but not women. These findings provide further support for the idea that competition-induced fluctuations in T may serve to modulate ongoing and/or future social behavior.