Abstract Systemic administration of drugs that augment 5-HT 2 activity generally induces ‘wet dog’ shaking (WDS) in rats. This suggests that the naturally occurring form of WDS seen in untreated rats may also serve as a behavioral index of 5-HT 2 receptor activation, during the performance of other behaviors. Indeed, spontaneously occurring WDS has previously been reported to be inversely related to male rat copulatory proficiency. In order to examine a potential central nervous system mechanism subsuming these behaviors, male rats were tested for WDS and sexual behavior after brainstem administration of the 5-HT 2 agonist DOI. Male Long-Evans rats were implanted with cannulae terminating in the region of the nucleus raphe obscurus/inferior olive, through which they received injections of DOI (0.1–10 μg). DOI produced a dose-dependent decrease in sexual behavior and concurrent increase in WDS. Pretreatment with the 5-HT 2 antagonist ritanserin effectively blocked the effects of DOI. The results suggest that WDS and copulatory behaviors are modulated by a shared brainstem substrate. It is possible that the results may be the behavioral concomitant of recently described brainstem cells, with bifurcating axons, that project to both the medial preoptic area and the cervical spinal cord.