The effects of a 5-HT2 receptor agonist, DOI (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane; 0.5 mg/kg), on the behavior of male rats at different ages when given alone or with different levels of testosterone, in the presence of sexually receptive and non-receptive females are presented. DOI increased mounting and/or mount plus thrusting behavior in adult males with receptive females. In pre-pubertal males, DOI increased the frequency of pursuit and genital sniffing in the presence of receptive females, but not of non-receptive ones, when no mounts or thrustings were recorded. In castrated rats treated with testosterone and tested with receptive females, DOI increased the frequency of thrusting behavior, but in castrated rats without testosterone treatment, DOI produced no change. DOI did not induce mounting in pre-pubertal or castrated rats without testosterone substitution therapy. These results suggest that DOI influences male sexual behavior through a neural system that is modulated by testosterone.