To investigate the changes in pituitary responsiveness to hypothalamic releasing hormones during the periparturitional period, women undergoing labor and vaginal delivery were stimulated with thyrotropin-releasing hormone. The percentage of incremental changes in prolactin and thyroidstimulating hormone were significantly lower in pregnant women at term than in nonpregnant control subjects. Evidence of augmented release of prolactin was disclosed after the onset of active labor. The percent increases in prolactin and thyroid-stimulating hormone were significantly higher at 24 hours post partum than at term. Administration of thyrotropin-releasing hormone to the gravid patient in active labor caused a brisk response in fetal thyroid-stimulating hormone, although the increase in fetal prolactin remained low. These findings suggest that the changes in serum triiodothyronine (T3) significantly influence the release of prolactin and thyroid-stimulating hormone in response to thyrotropin-releasing hormone during the periparturitional period.