鈥� Plasma thyrotropin (TSH) levels varied relatively little over minutes to days in the same individual not subjected to challenge with thyrotropin-releasing hormone (TRH). Thus, the TSH response to stimulation with TRH is not likely to be confounded by spontaneous changes of comparable degree. Variable numbers of depressed patients showed a blunted response to TRH stimulation of the pituitary, but the exact prevalence of this phenomenon remains to be clearly defined. In any case, the pituitary response to TRH stimulation was not correlated with the prevailing level of depression, nor did this response to an initial TRH challenge predict the degree of clinical change during a 15-day treatment during which three intravenous doses of 600 未g of TRH were given. Depressed patients, as do schizophrenics and normal patients, show diminished TSH responses to repeated challenges with TRH. Whether or not these three groups differ in regard to the rate at which the pituitary response can be abolished remains to be determined.