31P-Nuclear magnetic resonance (NMR) spectra were obtained in the living rat from 19 implanted prolactin-secreted pituitary tumours. Seven major peaks were found including those arising from the high energy phosphorus metabolites ATP and phosphocreatine. Intracellular pH of the tumours was measured and a relationship with prolactin secretion was observed, the highest plasma prolactin concentrations being associated with an intracellular pH greater than 7.18. Repeated NMR measurements in three tumours over periods of up to 21 days revealed progressive changes with age, shown by an increase in inorganic phosphate, a decrease in high energy phosphorus metabolites and a decrease in prolactin secretion. It is concluded that NMR spectroscopy provides a useful method of studying intracellular events which accompany hormone secretion in vivo.