The maternal plasma concentrations of GH and PRL increase dramatically upon the initiation of lactation in the rat. In light of the fact that these two hormones have evolved from one common precursor, we sought to determine if the neuroendocrine mechanisms regulating their concomitant increase during lactation are common or if they are functionally distinct. To evaluate this, lactating rats were passively immunized with antiserum raised against GHRH and then monitored for changes in GH and PRL secretion in response to suckling. On day 9 or 10 postpartum, pups were removed from their mothers at 0800 h. At 1100 h mothers were injected with normal rabbit serum (NRS) or GHRH antiserum (GHRH-ab). At 1400 h a control blood sample was drawn. Pups were then returned to their mothers, with subsequent blood samples drawn over the next 60 min. Plasma concentrations of GH significantly increased to 12.3 +/- 1.0 ng/ml (mean +/- SEM) in NRS-treated females after the return of the pups. In contrast, there was no change in GH concentrations in the females treated with the GHRH-ab. Plasma PRL concentrations rose approximately 200 ng/ml in both the NRS-treated animals and the GHRH-ab-treated ones. Body weight gains of the pups during the 60-min period of lactation were similar in both groups. These results suggest that the neuroendocrine mechanisms regulating the increases in GH and PRL during lactation are distinct and that GHRH is the hypothalamic factor responsible for the increase in GH. Furthermore, these results suggest that acutely interrupting the increase in GH secretion that occurs during lactation does not compromise nursing behavior and performance.