GH and PRL levels were measured by RIA in plasma samples taken from genetically obese and nonobese rats over a 6-h period at consecutive 15-min intervals. The mean GH level was 204 ng/ml for lean animals and 48 ng/ml for obese rats; the difference is significant (t = 5.8; P < 0.01). For the group of 5 lean rats, there were 10 GH peaks that exceeded the upper limit of the assay (800 ng/ml), whereas for the group of 6 obese rats, there were only 2 peaks that exceeded the upper limit. In some of the obese rats, peaks of very small amplitude were present. No differences were seen in PRL levels. The mean plasma PRL level was 3.6 ng/ml for lean animals and 3.3 ng/ml for obese rats. Abnormalities in GH in obese rats may be related to an imbalance between hypothalamic releasing and inhibiting factors or a defect in the pituitary.