It is now recognized that growth hormones (GH) can exert direct metabolic effects on adipose tissue from hypophysectomized or normal rats in vitro. This implies the presence of receptors for GH in this tissue. The interaction of 125I-labelled human and bovine GH has been studied therefore in isolated adipocytes from male and female rats. Binding of 125I-hGH was dependent on time (with equilibrium being reached by 2 hrs at 23 degrees C) and number of fat cells (binding being linear up to 1.5 x 10(6) cells/tube). Binding was partially reversible (half-time for dissociation being 4 hrs at 23 degrees C) and specific for GH being displaced by various growth hormones but not by lactogens or other hormones. Scatchard analysis revealed linear plots with an affinity (KA) of 0.8 x 10(9) M-1 and 24,000 sites per cell for adipocytes from 36-day old male rats. 125I-labeled bovine GH binding was somewhat lower, being reflected in a slightly reduced binding capacity. No obvious sex difference in hGH binding was evident but there was a quite striking age-dependence of binding, being low prior to weaning, reaching a peak between 24 - 36 days and then declining to low levels again in the adult. The demonstration of specific GH receptors in adipose tissue confirms the notion of direct actions of GH in this tissue and should now permit further studies of the relationship between GH binding and GH action.