OBJECTIVE: The purpose of this study was to examine the relationship between behavior and serotonin by using a nonhuman primate model of aggression and impulse control. METHOD: During a routine capture and medical examination, 26 adolescent male rhesus macaques (Macaca mulatta) were selected as subjects from a free-ranging population of 4,500 rhesus monkeys inhabiting a 475-acre sea island. Physiological data were obtained from 22-23 of the subjects. Blood and CSF samples were obtained, and each subject was fitted with a radio transmitter collar for rapid location. The subjects were released into their social groups, and quantitative behavioral observations were made over a 3- month period. RESULTS: CSF 5-hydroxyindoleacetic acid (5-HIAA) concentrations were inversely correlated with "escalated" aggression, i.e., a measure of more intense or severe aggression as defined by the ratio of chases and physical assaults to all aggressive acts. CSF 5- HIAA concentrations were significantly lower in those subjects who showed evidence of physical wounding than in subjects with no wounds. Low CSF 5-HIAA concentrations were also correlated with greater risk- taking as determined by an analysis of leaping behaviors in the forest canopy. The ratio of long leaps (leaps that traversed the longest distances at dangerous heights) to all leaps was negatively correlated with CSF 5-HIAA concentrations. CONCLUSIONS: Adolescent male rhesus macaques with low CSF 5-HIAA concentrations are at risk for 1) exhibiting more violent forms of aggressive behavior and 2) loss of impulse control as evidenced by greater risk taking during movement through the forest canopy.