Thirty-one human subjects were administered a neuroendocrine challenge with the 5-HT1a agonist ipsapirone after completing six sessions of a laboratory measure of aggression, the Point Subtraction Aggression Paradigm (PSAP), in order to determine if a laboratory measure of aggression was related to serotonin function. Subjects who showed more aggressive responding on the PSAP (n = 11) had a significantly blunted temperature response to ipsapirone compared to those with less aggressive responding (n = 20). There was no difference between the two groups on the cortisol response to ipsapirone. This study supports a relationship between serotonin function and aggression as measured in the human laboratory, similar to the well-documented association between self-reported aggression and serotonin.