The nature of the inhibitory action of mammalian prolactin on the development of bullfrog tadpoles was investigated. Tadpoles injected ip with 20 [mu]g of thyroxine (T4) and 2 or 4 mg of ovine prolactin (NIH [National Institute of Health]-P-S-7) underwent metamorphic changes in parallel with T4-injected controls. By contrast, tadpoles injected with 4 IU of bovine thyrotropic hormone (TSH) and either 4.0 or 0.4 mg of prolactin entered into developmental stasis after showing early signs of metamorphic climax, while TSH-injected controls completed the various metamorphic steps, including tail resorption. Histological examination of the thyroids of animals treated with both TSH and prolactin revealed an extremely high degree of hypertrophy and hyperplasia, with more pronounced depletion of the colloid than in TSH-injected controls. It is concluded that in these experiments prolactin acts at the level of the thyroid and that it has goitrogenic properties.