THYROTROPHIN-RELEASING hormone (TRH) produces behavioural effects in experimental animals1 and may have psychoactive properties in man2-5. Winokur and Utiger6 and Brownstein et al. 7 have described the distribution of TRH in rat brain, suggesting that TRH has a modifying role in synaptic functions in addition to its effect on the pituitary. The behavioural effects of endogenous TRH release in the brain are not known; however, Prange et al.8 noted that the systemic administration of TRH to pentobarbital-anaesthetised rats resulted in lacrimation, paw tremor and a peculiar shaking movement of the head and trunk. These behavioural effects were also obtained in partially anaesthetised rats following intracisternal injection of 10 μg TRH per animal. The TRH-induced shaking was particularly interesting because we have observed this behaviour as a characteristic sign of morphine abstinence in the anaesthetised rat 9. Here, we have studied the central sites of TRH-induced shaking to determine if these sites parallel the endogenous distribution of TRH in the rat brain and also to determine if these sites correspond to brain areas where morphine withdrawal shakes are obtained.