The effect of the peptides TRH and vasoactive intestinal polypeptide (VIP) on phosphate-bound energy of GH3 tumor cells in vivo was investigated using the noninvasive technique of topical 31P-nuclear magnetic resonance spectroscopy. VIP (4.0 micrograms/100 g BW) caused a reduction in the high energy metabolite phosphocreatine and an increase in inorganic phosphate. In the same experiments (n = 9), VIP stimulated PRL secretion to a value of 146% +/- 13 of basal concentrations. The change in the inorganic phosphate-phosphocreatinine ratio had a positive correlation with the increase in PRL secretion (P less than 0.001), indicating a VIP-induced energy consumption for a process related to PRL secretion. In separate experiments, in vitro, it was confirmed that VIP increased energy consumption by demonstrating an increase in glycogenolysis by the pituitary tumor cells. TRH also increased PRL secretion to 145% +/- 23 of basal values but, in contrast, did not induce changes in energy metabolites detectable by nuclear magnetic resonance. It is concluded that VIP and TRH promote PRL release through different stages in the secretion process.