In this study, we found that methylmercury chloride inhibited the binding of 3H-QNB to muscarinic receptor of rat brain tissue in vitro with IC50 values of 0.0137 +/- 0.0037mol/L. It also decreased the densities of muscarinic receptor and affinity to 3H-QNB. In vivo test system, the rats were exposed to methylmercury chloride (ip) 0, 1.5, 2.5, 3.5 mg/kg.d on the 7-12th day of gestation. On the 7, 14 and 21st day after birth, the offsprings were killed, and the cerebrum and cerebellum were immediately dissected on ice and homogenizated. The ligand assays were carried out. The result showed the effect of inhibition on the binding to cerebrum and cerebellum with significant correlation.