In this study, the authors set out to determine the presence of M3 muscarinic receptors in rat striatum by examining the binding of [3H]N-methylscopolamine ([3H]NMS) to striatal membranes and its displacement by antagonists with different affinity for M1 and M3 muscarinic receptors -pirenzepine; 4-diphenylacetoxy-N-methylpiperidine methiodide, 4-DAMP; and the p-fluoro analog of hexahydro-sila-difenidol, pFHHSiD). The specific binding of [3H]NMS to membranes from rat striatum (551 +/- 40 fmol.mg prot.-1, KD 0.11 +/- 0.01 nM) was displaced in a concentration-dependent manner by all three antagonists tested. Inhibition curves best fit to a single-site model for 4-DAMP (pKi 9.1 +/- 0.1), whereas for both pirenzepine and pFHHSiD, the best fit was to the two-site model. The pKi values for the high-affinity (8.0 +/- 0.2) and low-affinity (6.7 +/- 0.2) components for pirenzepine-mediated inhibition of [3H]NMS binding corresponded to those reported for M1 and M3 receptors, respectively. The pKi values for the high-affinity (7.7 +/- 0.1) and low-affinity (7.1 +/- 0.2) components for pFHHSiD inhibition were in good agreement with those reported for M3 and M1 receptors, respectively. Altogether, these results indicate the presence in rat striatum of both M1 and M3 muscarinic receptors. These findings might be relevant to the design and use of muscarinic antagonists in the treatment of neurological disorders such as Parkinson's disease.