Microinjection of neurotensin (NT) into the ventral tegmental area (VTA) of the rat produced a dose-dependent increase in spontaneous motor activity. The NT-induced hyperactivity consisted of an increase in exploratory behaviors, such as locomotion, rearing and sniffing, and a decrease in sleep or resting. The structural specificity of this response was demonstrated by microinjecting NT analogues endogeneous neuropeptides into the VTA. The fact that high levels of immunoreactive NT have been demonstrated in the VTA indicates that the observed behavioral effects may reflect an underlying physiological action by endogenous NT.