The extent to which trait factors (baseline testosterone concentrations, trait dominance) and state factors (change in social status, change in testosterone concentrations) would predict reactive aggression in a subsequent task that involved provocation was examined in 99 participants (39 men and 60 women). Participants first competed in same-sex dyads on a Number Tracing Task for which the outcome (win or loss) was rigged. After the competition, participants performed the Point Subtraction Aggression Paradigm (PSAP), a behavioural measure of reactive aggression against an opponent (actually a computer program). Trait dominance predicted baseline testosterone in men, but not women, and men made more aggressive responses than did women. Baseline testosterone concentrations did not predict aggressive behaviour in either men or women. Winners and losers did not differ in competition-induced change in testosterone. However, change in testosterone concentrations predicted aggressive responses in the PSAP for men in the loss condition, and aggressive responses were made at a cost to obtaining reward points. For men in the win condition, aggressive responses were predicted by an interaction between trait dominance and change in testosterone concentrations. These findings suggest that situational changes in testosterone concentrations modulate future aggressive behaviour in men.