The muscarinic cholinergic agonist bethanechol (0.25-1.0 micrograms) injected bilaterally into various parts of the rat neostriatum induced a tonic electromyogram (EMG) activity in the gastrocnemius muscle which is considered to be a measure of limb rigidity. This tonic EMG activity was found to be dose-dependent and muscarine-specific since it could be blocked by coadministration of the muscarinic antagonist N-methylscopolamine (1.0 micrograms). Tonic EMG activity of comparable amount was observed after injections of bethanechol (1.0 microgram) into all regions of the neostriatum but not into the globus pallidus, thalamus, zona incerta or cortex. The tonic EMG activity induced by intrastriatal injection of bethanechol (1.0 microgram) was abolished by a subsequent injection of the GABAmimetic drug muscimol (25 ng) into the posterior part of the substantia nigra pars reticulata suggesting that bethanechol-induced limb rigidity is mediated via impairment of GABAergic transmission within the substantia nigra pars reticulata.