In order to study the interactions between serotonergic mechanism and electrical stimulation of the mesencephalic central gray substance, rats were trained to lever-press for terminating aversive electric stimuli applied at the Periaqueductal gray and adjoining tectum of the mesencephalon. Experimental sessions consisted of 40 discrete escape trials of a maximum of 30 sec duration, separated by 30 sec intervals. Dose-effect curves of two tryptamine antagonists, cyproheptadine and methysergide, as well as of the benzodiazepine minor tranquilizer, chlordiazepoxide, on average escape latencies and on frequency distribution of individual latencies were determined. Doses of 3 to 10 mg/kg of cyproheptadine decreased average latencies of escape responding in six of eight rats studied. Doses of 10 and 30 mg/kg of methysergide also facilitated escape responding in one of three rats. In contrast, doses from 1 to 10 mg/kg of chlordiazepoxide, that cause little sedation or ataxia, produced dose-dependent increases in escape latencies. Furthermore, doses of 5.6 and 10 mg/kg of chlordiazepoxide partially blocked escape responding. The facilitatory effects of the tryptamine antagonists suggest that escape behavior is inhibited by brain tryptaminergic mechanisms, whereas the specific depressant effect of chlordiazepoxide on escape from Periaqueductal gray electrical stimulation suggest that this region may be involved in the antianxiety action of benzodiazepines.