1. The pharmacological effects of TRH are compared to those produced by d-amphetamine in an attempt to elucidate the mechanisms underlying the activity of this endogenous peptide. 2. Although numerous amphetamine-like actions have been attributed to TRH, several differences have been noted between these compounds and are discussed. 3. At present, it is impossible to propose a single mechanism of action to explain the behavioral effects of TRH.