1. 10 male cocaine dependent patients and 10 sex matched controls were administered several behavioral measures of aggression including the Buss-Durkee Hostility Inventory, and The Brown-Goodwin Life History of Aggression. 2. All subjects were also administered a buspirone neuroendocrine challenge as a measure of serotonin function. 3. The cocaine dependent subjects were significantly more aggressive than the controls. 4. There was a significant correlation between the growth hormone response to buspirone and behavioral measures of aggression in the cocaine dependent subjects, but not in the controls. 5. There was no difference in the overall growth hormone response between the controls and cocaine dependent subjects, possibly due to differences in metabolism of buspirone. 6. This study supports a role for serotonin in aggression in cocaine dependent subjects.