Prolactin (PRL) and growth hormone (GH) secretions in mice rendered obese by the administration of gold thioglucose (GTG) are abnormal. The objective of the present experiments was to determine whether the effects were related to the drug or to the resultant obesity. Perphenazine-induced PRL release in normal mice and in GTG-injected non-obese mice was compared to that of GTG-injected obese mice after the initial development of obesity, after body weight reduction by diet control and after the resumption of obesity by ad lib. feeding. The GTG-injected mice which did not become obese had greater (50%) than normal levels of serum PRL following perphenazine stimulation in 2 of 3 experiments. This suggested that the injection of GTG directly affected the control mechanism for PRL secretion, but that the abnormal PRL secretion was probably not the cause of obesity that develops after GTG treatment. Perphenazine-induced PRL levels in mice rendered obese with GTG were much greater (2-3 times higher than normal). However, the unusually high levels of PRL were totally abolished when the body weights of these mice were brought down to normal by dietary restriction. Conversely, when obesity was permitted to recur by giving the mice free access to food, PRL levels reverted back to the original obese pattern. The concentrations of GH were usually lower than normal in GTG-obese mice, and these levels were also more often associated with the development of obesity than with the injection of GTG. The data show a marked influence of obesity on the control of PRL and GH secretions in the mouse.