Prior research [van Honk J, Tuiten A, Verbaten R, van den Hout M, Koppeschaar H, Thijssen J, de Haan E. Correlations among salivary testosterone, mood, and selective attention to threat in humans. Horm Behav 1999;36(1):17-24; van Honk J, Tuiten A, Hermans E, Putman P, Koppeschaar H, Thijssen J, Verbaten R, van Doornen L. A single administration of testosterone induces cardiac accelerative responses to angry faces in healthy young women. Behav Neurosci 2001;115(1):238-42.] showed relationships in humans between testosterone (T) and vigilance to facial expressions of anger, which are considered signals of an impending dominance challenge. In Study 1, we used a differential implicit learning task (DILT) (see [Schultheiss OC, Pang JS, Torges CM, Wirth MM, Treynor W. Perceived facial expressions of emotion as motivational incentives: evidence from a differential implicit learning paradigm. Emotion 2005;5(1):41-54.]) to investigate the degree to which subjects find anger faces reinforcing. In the DILT, separate sequences of actions were paired with presentations of anger faces, neutral faces or a blank screen. After training, performance on the three sequences was measured in the absence of face stimuli. Saliva was collected for T measurement. Higher T predicted better learning on sequences paired with sub-threshold (i.e., presented too fast for conscious awareness) anger faces, suggesting that T is related to reinforcing qualities of these faces. In Study 2, we examined whether morning or afternoon T better predicted attention and vigilance to anger faces. Participants were tested at 9:00 and 15:00. At each session, saliva was collected for T measurement, and participants completed a Stroop task and a dot-probe task [Mogg K, Bradley BP, Hallowell N. Attentional bias to threat: roles of trait anxiety, stressful events, and awareness. Q J Exp Psychol A 1994;47(4):841-64.] with facial expression stimuli. Morning (peak) T was a better predictor of responses to anger faces than afternoon T. Morning T predicted greater Stroop-like interference to sub-threshold anger faces, as well as attentional orienting away from sub-threshold anger faces. These effects were not present for joy faces or for supraliminal anger faces. T may generally decrease aversion to threatening stimuli, and/or may specifically facilitate approach towards signals of dominance challenge.