In this paper, the authors summarize the results of 3 randomized, double-blind within-subjects studies examining the contributions made by serotonin release, 5HT2A receptors and D2 receptors in producing the subjective effects of MDMA in humans. The authors also summarize in vitro and rodent studies supporting the significance of MDMA-induced norepinephrine release, and possible actions on 5HT1a, 5HT1b, acetylcholine and histamine receptors. Using their own studies and rodent studies, they suggest that activation produced by higher doses of MDMA is the result of direct or increased activity at 5HT2 receptors, while activity at lower doses is related to 5HT1A or 1b receptor activity. The paper ends with an exhortation for further research into the relationship between the subjective effects of MDMA and the possible neurochemical underpinnings of these effects.