Intracerebroventricular (i.c.v.) injection of neurotensin (NT) induced catalepsy in mice at doses greater than or equal to 0.02 microgram. The cataleptic effect progressively increased, reaching a maximum at approx. 2 hr after injection. In contrast, the hypothermic effect of neurotensin reached a maximum 1 hr after the injection, and was declining at 2 hr. Not all mice that showed hypothermia also showed catalepsy, and some mice showed catalepsy without hypothermia. Catalepsy induced by intracerebroventricular injection of neurotensin was not significantly correlated with the hypothermia. Furthermore, oxotremorine induced hypothermia without catalepsy. Thus, several lines of evidence indicate that the catalepsy induced by neurotensin is not the consequence of the neurotensin induced hypothermia. Thyrotropin releasing hormone (TRH), injected either intracerebroventricularly with neurotensin, or intraperitoneally before neurotensin abolished the hypothermia but only diminished the catalepsy scores. The cataleptic effect of neurotensin is consistent with its other neuroleptic-like activities.