Bethanechol chloride (5-25 micrograms), when injected into the substantia nigra pars reticulata (SNR) of rats, produced muscular rigidity in a dose-dependent way, and in addition, catalepsy and ipsilateral posture. The effects of bethanechol in the dose of 25 micrograms were prevented by co-administration of 10 micrograms scopolamine hydrochloride. Injections of 25 micrograms bethanechol or 10 micrograms scopolamine into the reticulata only slightly affected the muscular rigidity produced by 15 mg/kg i.p. morphine hydrochloride. The results suggest that muscarinic cholinergic mechanisms in the substantia nigra pars reticulata, although effective by themselves, affect by expression of at least one striatal functional alteration, the muscular rigidity, in a less effective way than GABAergic or endogenous opioid mechanisms do.