This study examines sexual behavior, serotonin turnover in the central nervous system, and testosterone in free-ranging non-human primates. Study subjects were 33 young adult male rhesus macaques (Macaca mulatta) living in naturalistic social groups on a 475-acre South Carolina barrier island. Blood and cerebrospinal fluid (CSF) samples were obtained during random trappings, and the subjects were located for observation by radio telemetry. Quantitative behavioral samples totaling 203 observation hours were taken during two mating seasons (September through January) in 1994 and 1995. Control observations (65 h) on 13 subjects were also taken during the non-mating seasons in 1994 and 1995. The results indicate that CSF 5-hydroxyindoleacetic acid (5-HIAA), CSF testosterone, and plasma testosterone concentrations increase significantly during the mating season. During the mating season, there were significant increases in high intensity aggression, low intensity aggression, grooming behavior, and heterosexual mounting. In the mating season, CSF 5-HIAA was significantly correlated with several sociosexual behaviors: consorts per hour, heterosexual mounts per hour, and inseminations per hour. In contrast to previous findings from the non-mating season, CSF 5-HIAA was not correlated with any measures of aggression or sociality, although during consorting, CSF 5-HIAA was positively correlated with grooming. From these findings, we conclude that the lack of correlation between intense and severe aggression and CSF 5-HIAA in the mating season may reflect the use of high intensity aggression in 'normative' male-male competition over access to reproductively active females. We also conclude that CNS serotonin turnover is positively correlated with sexual competence, i.e. males with low CSF 5-HIAA concentrations are less sexually competent than males with higher concentrations.