Dehydroepiandrosterone, a mammalian glucose-6-phosphate dehydrogenase inhibitor, prevented Avy/a mice from becoming obese. Decreased accumulation of triacylglycerol accounted for a large portion of the weight difference between treated and control Avy/a mice. Hepatic lipogenesis as measured by 3H2O incorporation into total lipid was less in the dehydroepiandrosterone-treated mice. Dehydroepiandrosterone did not suppress appetite and had no apparent toxic effects at the doses used, and its weight controlling effects were reversible upon withdrawal of treatment.