The diurnal rhythms of body temperature and serum thyrotrophin (TSH) levels in euthyroid individuals behaved inversely to one another. An artificially induced rise in the body temperature of these individuals was accompanied by a fall in serum thyrotrophin levels, the amplitudes of their respective rhythms decreasing simultaneously. There was a marked correlation between the degrees of change in body temperature and circulating thyrotrophin levels respectively during warming. In normal subjects manipulation of the diurnal rhythm of body temperature brings about inverse alterations in the thyrotrophin rhythm.