To investigate the contrasting effects of leptin on ovarian steroidogenesis, B6 mice pharmacologically treated with leptin for 16 days on CD were used to model hyperleptinemia, while homozygous ob/ob (−/−) mice with genetic leptin deficiency, also on a CD, were used to examine the effects of obesity in the absence of leptin. The gene discussed is LEP; the disease is obesity due to melanocortin 4 receptor deficiency.