The effect on body temperature of lateral ventricular injection of dopamine was studied in control rats and in rats rendered dependent on morphine by implantation of a subcutaneous pellet. Dopamine induced a significantly greater hypothermie response in animals of the morphine-dependent group when injected 72 hr after pellet implantation. The increased sensitivity of the morphine-dependent animals is probably the result of changes at the dopaminergic synapse; whether this is due to an increase in receptor sensitivity or is a reflection of an increase in receptors as a result of presynaptic inhibition of transmitter release is discussed.