The role of the nucleus accumbens in the generation of the signs of morphine-induced "catatonia" namely akinesia, catalepsy and muscular rigidity, was studied in rats. Morphine was injected into the nucleus accumbens and either spontaneous locomotor activity or catalepsy or activity in the electromyogram of the gastrocnemius-soleus muscle, signalling the appearance of rigidity, were recorded. Unilateral injections of 5 micrograms of morphine induced a decrease of locomotor activity and weak catalepsy; 15 micrograms of morphine completely abolished locomotor activity (akinesia) and produced a very pronounced catalepsy. All these effects were antagonized by naloxone (2 mg/kg i.p.). Injections of morphine into the nucleus accumbens did not induce muscular rigidity. In contrast, injection of morphine (15 micrograms) into the head of the caudate nucleus, which induced a pronounced muscular rigidity, did not noticeably alter the locomotor activity nor did it produce catalepsy. Our results suggest that 1) the nucleus accumbens is relevant for systemically administered morphine to produce akinesia and catalepsy, but is not noticeably involved in the development of muscular rigidity; 2) they provide evidence that morphine-induced catalepsy is largely due to a strong akinesia, and that muscular rigidity, observed after morphine administration, does not contribute to positive scores in the catalepsy test.