The objective of this study was to investigate changes in the secretion of prolactin (PRL) and growth hormone (GH) occurring during a 24 h period in the mouse. Adult female mice of the C57BL/St strain and male mice of the C3H/St strain, maintained on a 14 h light and 10 h dark schedule, were used. Serum and pituitary concentrations of PRL and GH were measured by radioimmunoassay in samples collected by decapitation at hourly intervals through 24 h. Serum PRL concentrations in female mice averaged higher during the daylight hours and lower at night. However, the pattern was just the opposite in males: the values were lower during the day time and higher and variable during the night. Pituitary PRL levels dropped significantly after the onset of the dark phase in mice of both sexes. Serum GH concentrations of female mice did not fluctuate significantly with the time of the day, but those of male mice displayed a distinct flux: the levels were low from 0800 h until 1500 h, began to rise in the afternoon, and remained relatively high throughout the night. Pituitary levels of GH did not change with time in mice of either sex. The data suggest the existence of daily rhythms in the secretion of PRL and GH in mice, with marked differences related to sex. In general, the changes were most pronounced for serum PRL in females and for serum GH in males.