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The “winner–loser effect” refers to a phenomenon in testosterone research where the outcome of a social competition induces increases (wins) and/or decreases (losses) in testosterone levels. Here, we sought to test to what extent changes in testosterone occur in response to gambling behavior. More specifically, we hypothesized that the winner–loser effect would extend to slot machine gambling as a solitary (noncompetitive) gambling activity in players who “anthropomorphized” the slot machine, thus treating the machine as a human opponent. Male participants (n = 113) were recruited into a quasi-experimental design involving 15 min of authentic slot machine gambling, incentivized by a $10 cash bonus for participants who finished in profit. In addition to salivary measures of testosterone, salivary cortisol and self-reported anthropomorphization of the slot machine were tested as potential moderators. Contrary to predictions, winning and losing slot machine sessions did not exert significant differential effects on testosterone, and this pattern was not moderated by cortisol levels or slot machine anthropomorphization. Exploratory analyses tested relationships between subjective gambling experiences in the sessions and testosterone change. Higher positive affect and flow predicted greater testosterone declines from pre- to postgambling. The testosterone results add to a growing literature on the boundary conditions of the winner–loser effect and inform future studies on testosterone reactivity in relation to gambling and disordered gambling. The tendency to anthropomorphize slot machines is a neglected cognitive distortion in gambling that merits further study.