The aim of this study was to examine the effect of acute administration of ( f)3,4-methylenedioxymethamphetamine (MDMA) (20 mg/kg, i.p.) on colonic temperature, serum corticosterone and brain biogenic amine concentrations in group-housed male and female rats and to determine if there are any gender differences in susceptibility to MDMAinduced changes. Prior to treatment, basal colonic temperatures were higher in female than in male rats (p<0.05). Temperatures were significantly increased 30 min following MDMA (20 mg/kg, i.p.) in male rats (p<O.O5), and 120 min following M D M A in female rats (p<0.05), and were still elevated 240 min following M D M A administration in both sexes. Basal corticosterone concentrations were also higher in female than in male rats (p<O.OS), prior to treatment. Serum corticosterone concentrations were significantly elevated in both male and female rates 15 min after MDMA administration (p<O.OOI), and remained elevated for up to 4 h following M D M A (p<O.OOl and p < 0.05 respectively). 5-Hydroxytryptamine (5-HT) and 5-hydroxy-indole-3-acetic acid (5-HIAA) concentrations were significantly depleted in the frontal cortex, hypothalamus and striaturn of both male and female rats. However, pineal S-€IT was resistant to depletion by MDMA in both male and female rats. The 5-HIAA concentration was significantly reduced in the pineal gland of female rats, probably due to decreased catabolism of pineal 5-HT. There was a profound decrease in the ratio 5-HT/5-HIAA in the frontal cortex but no changes occurred in these ratios in the striaturn, hypothalamus or pineal gland in either male or female rats. Dopamine (DA) concentrations were increased and 3,4-dihydroxyphenyl-acetic acid (DOPAC) levels decreased in the striatum in both male and female rats 4 h after MDMA administration. The iioradrenaline (NA) content of the hypothalamus was increased in male rats 30,60 and 120 min post MDMA (p<O.O5) but no changes were noted in the hypothalamus of female rats. This study indicates that there are slight gender differences in susceptibility to MDMA-induced changes in rats.