Group-housed mice were given at least 2 and no more than 3 doses of 0, 5 or 10 mg/kg MDMA over 3 days in a study comparing the acute and long-term effects of MDMA exposure at various points in development. Mice received vehicle or MDMA at 28, 38 or 52 days old (calculated as early, middle or late "adolescence.") When measured with the hot plate test, MDMA increased pain threshold (reduced forepaw licking) for mice in all conditions, with analgesic effects seen after 5 mg/kg MDMA. Increase in pain threshold was greater for mice given MDMA at 38 and 52 days than for mice given MDMA at 28 days. When placed in a cage with an unfamiliar conspecific, approximately 28 to 52 days after the last dose of MDMA (at 80 days for all mice), mice treated with MDMA during "early" and "late" adolescence showed increased social interaction and environmental exploration. Mice given MDMA in "middle" adolescence showed reduced social interaction. In both cases, effects were greater after treatment with 10 mg/kg MDMA than after 5 mg/kg. MDMA neither decreased nor increased aggression in any of the mice. MDMA treatment increased self-grooming, regardless of time of treatment. Mice given 10 mg/kg MDMA demonstrated more locomotor activity in an unfamiliar test environment than did mice given vehicle or 5 mg/kg MDMA. When given a second challenge with 5 mg/kg MDMA, mice in all conditions spent more time in a novel area of a test environment (after familiarization) than in the familiar area, and they were less active in the novel compartment. Mice previously given 10 mg/kg MDMA were more active after the second MDMA challenge than mice given vehicle or 5 mg/kg MDMA, regardless of time when drug was administered. When mice were sacrificed immediately after the second MDMA challenge and after assessment of novelty seeking, increased hypothalamic 5-HT and 5-HIAA was seen immediately after MDMA administration. Mice who had not received MDMA before sacrifice but had been previously treated with either 5 or 10 mg/kg MDMA had lower hypothalamic 5-HT and 5HIAA. There were no differences due to age of exposure for mice given 10 mg/kg MDMA, but 5 mg/kg was only associated with lower hypothalamic 5-HT and 5-HIAA when given at 38 or 52 days. Overall, study findings suggest that sensitivity to both the acute and the neurotoxic effects of MDMA changes with developmental period. However, it is difficult to relate findings in mice to effects in humans, mice develop far more rapidly than humans, and also because potential neurotoxicity after MDMA affects dopaminergic neurons in adult mice, not serotonin neurons. Other studies assessing anxiety after MDMA (Fone 2002; Mechan 2002; Morley 2002) have found long-term effects of increased anxiety and decreased anxiety, suggesting a complex relationship between MDMA, anxiety and social interaction.