A consistent effect of repeated exposure to 3,4 methylenedioxymethamphetamine (MDMA) is a decrease in the tissue levels of serotonin (5-HT). A variety of behavioural and neurochemical tests were conducted to determine whether the tissue deficits were accompanied by an increased sensitivity of the 5-HT1a autoreceptor. Tests were conducted 2 weeks following MDMA exposure (four injections of 10.0 mg/kg, IP, administered at 2-h intervals in a single day). The response to the 5-HT1a agonist, 8-OHDPAT (0.003-0.5 mg/kg, SC), was assessed using lower lip retraction (LLR), hypoactivity, and 5-hydroxytryptophan (5-HTP) accumulation following decarboxylase inhibition. The 8-OHDPAT produced a dose-dependent increase in LLR and hypoactivity, but these effects were comparable for MDMA and saline pretreated groups. MDMA decreased tissue levels of 5-HT and the accumulation of 5-HTP, but these effects were not reflected in the changes in autoreceptor sensitivity. The data suggest that the decrease in tissue levels of 5-HT produced by MDMA is accompanied by a decrease in tryptophan hydroxylase activity but cannot be explained by supersensitivity of the 5-HT1a autoreceptor.