SummaryBasal levels of GH and PRL in the serum and pituitary gland of normal (Pg/ Pg) and pygmy (pg/pg) (VY X C3H) F-l hybrid mice were measured by homologous radioimmunoassays. The serum concentrations of GH and PRL in pygmy mice at approximately 90 days of age were found to be no different from those in corresponding normal controls. The pituitary concentrations of the two hormones, however, were substantially subnormal in pygmy mice. Perphena-zine injection failed to stimulate PRL release by pygmy (pg/pg) pituitaries, in contrast to the marked increase in serum PRL levels of the normal Pg/Pg mice. Daily administration of 100 μg of ovine PRL for 8 weeks produced none or very little increase in the body weight of pygmy mice. These results suggest that the pg mutation induces a multifaceted change affecting the synthesis, release, and function of growth hormone and prolactin.The ovine PRL used in these studies was supplied by the Pituitary Hormone Distribution Program of the National Institute of Arthriti...