The aim of this study was to investigate the pattern of growth hormone (GH) secretion during a 24-hour period in the rat, in order to determine whether differences in plasma GH between male and female animals could be demonstrated. The daily fluctuations of plasma GH were characterized by three major bursts of secretion occurring at 09.00, 16.30 and 21.00 h in both sexes. In females, the three peaks were of the same extent, while in males the night peak was greater than the other two. Female rats maintained higher GH levels than males at 09.00, 12.00 and 16.30 h, while no differences between both sexes were found at all other times. These results show that sampling time seems to be of great importance in evaluating the differences in plasma GH concentration between experimental groups in which there could be marked differences in the daily patterns of hormone secretion.