The purpose of this study was to determine the effect of rendering newborn male Sprague-Dawley rats hypothyroid with 131I on development of the hypothalamic-pituitary axis. Hormonal responses to thyrotropin-releasing hormone (TRH). haloperidol (H), and luteinizing hormone-releasing hormone (LHRH) were determined at 9, 15, 30 and 75 days of age in hypothyroids and controls. Basal and TRH-stimulated thyroid-stimulating hormone (TSH) levels were higher in the hypothyroids, but the developmental patterns of basal TSH secretion and TSH reserve were parallel in the two groups, with maximal TSH secretion occurring at 30 days. Basal prolactin (PRL) and PRL responses to TRH and H were less in the hypothyroids at all ages. Basal LH and LH responses to LHRH were greater in the controls at days 9 and 15, but the hypothyroid rats displayed greater basal and stimulated LH secretion at 30 days and values similar to controls at 75 days of age. These results suggest that development of the hypothalamic-pituitary axis is altered in rats rendered hypothyroid shortly after birth.