Experiments were performed to ascertain circadian fluctuations in plasma levels of LH and FSH in juvenile and adult male mice. Animals under natural lighting (11 h day/13 h night) were killed at 1-hour intervals over a 24-hour period. There were large variations in plasma LH concentrations between animals sacrificed within each killing period. Baseline LH levels (values lower than 60 ng/ml) showed a significant 24-hour periodicity in adult males. FSH concentrations exhibited significant diurnal variations in juvenile and adult males. There was significant influence of age on the temporal pattern and 24-hour mean plasma hormone levels.