1. Daily fluctuations in the plasma concentrations of thyroid stimulating hormone (TSH) and growth hormone (GH) have been studied in six normal males by using a 24 h continuous blood-sampling technique and a highly sensitive and specific radioimmunoassay for TSH. 2. In all subjects the plasma concentrations of the two hormones were increased at night. The TSH concentrations rose approx. 2 h before the onset of sleep but the peak concentration coincided with the GH peak 2 h after the commencement of sleep. In three subjects who slept in the afternoon, additional peaks of GH activity were found and in two cases peaks of TSH activity also occurred. 3. The mean data from all subjects showed a significant circadian variation of TSH secretion with a maximum between 2 a.m. and 4 a.m. and a minimum between 6 p.m. and 8 p.m.