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The uptake of ¹²⁵I-labelled ovine prolactin in the bullfrog kidney was studied by autoradiography. Five minutes after the intracardiac injection of ¹²⁵I-labelled prolactin, no labelling was detected in the kidneys of premetamorphic tadpoles or in animals whose forelimbs had just emerged (climax tadpoles). After 15 min, a few isolated tubules were labelled in the premetamorphic tadpole kidneys and many kidney tubules were labelled in climax tadpoles. In the froglet kidneys, extensive labelling was seen by 5 min after injection, and only distal tubules seemed to be consistently unlabelled. The radioactive label was displaced by an excess of unlabelled ovine prolactin, but not by ovine growth hormone or bovine LH. These findings provide the first morphological demonstration of specific binding of prolactin by the nephric tubules of an amphibian. They show that the receptors, present in small numbers in the premetamorphic stage, proliferate during the later stages of metamorphosis. The change in receptor density involves both an increase in the number of tubules having receptors and in the density of receptors per tubule.