The nature and tissue distribution of prolactin receptor (PRL-R) mRNA in both male and female rats was studied. A single mRNA species of 2.2 kb was identified in the liver, kidney, adrenal, prostate, lactating mammary gland and ovary but not in the male lung, heart, skeletal muscle, thymus, adipose tissue or brain. There were distinct and contrasting sex differences in abundance of PRL-R mRNA in some tissues: liver (female much greater than male), kidney and adrenal (male much greater than female). A mRNA species of 4 kb was occasionally detected in the male adrenal and female liver. Given previous reports on the effects of thyroid status on PRL binding, the effects of thyroxine (T4), propylthiouracil (PTU) or combined treatment on PRL-R mRNA were assessed. In the male rat, PTU treatment markedly increased (three- to fourfold) PRL-R mRNA in the liver but decreased it (approximately 50%) in the kidney. These changes were reflected in similar changes in lactogenic binding activity. T4 or PTU treatment increased PRL-R mRNA in the prostate, with no obvious changes in binding. No major changes were seen in adrenal glands. In the female rat, PTU had little effect on PRL-R mRNA in any tissue, although binding of 125I-labelled lactogen was decreased in both the liver and kidney. There was an unexpected threefold rise in PRL-R mRNA in the female kidney following combined T4 and PTU treatment. Overall, there was a quite close correlation between the effects of thyroid status on PRL-R mRNA levels and specific lactogenic binding to membranes prepared from the same tissue samples. These studies provide data on the tissue distribution and size of PRL-R mRNA in rats and suggest a novel and complex tissue- and sex-dependent regulation by thyroid hormone.