Plasma levels of exogenous TRH were measured in eleven normal subjects. They were assessed as to their possible associations with the TRH disappearance rate in vivo, with the TSH and prolactin response, and with the behavioral (and side) effects of TRH. Max. levels of plasma TRH occurred consistently two minutes after injection of TRH (0.5 mg). They ranged from 8.1 to 75.1 ng/ml and were negatively correlated with the in vivo TRH disappearance rate. The latter, in turn, was positively correlated with such factors as height and body surface. Max. plasma TRH levels were not related to the magnitude of the TSH or prolactin response, or to the occurrence of behavioral effects. Nor were they related to the TRH-induced side effects although these effects occurred during the time of increasing TRH plasma levels.