The existence of circadian variations in serum TSH levels was investigated in 8 normal subjects. The analysis of the data indicated the presence of 2 different reproducible phenomena, a circadian rhythm characterized by an important acrophase and lower amplitude higher frequency variations, with period between 1 and 3 hours. It is suggested that neural mechanisms are at the origin of both the high frequency variations and the circadian phenomenon, this latter being related to the sleep-wake cycle. A significant difference, between nocturnal and diurnal mean TSH levels was observed in women, but not in men.