Using in vitro autoradiography with a digital autoradiography system and radioreceptor assay, the distribution and the binding characteristics of the muscarinic cholinergic receptors(mAChR) were studied in regions of rat brain. Radioreceptor assay revealed that mAChR could be measured with saturation binding assay in the brain and heart homogenates. No difference in Kd or Bmax of the brain or heart was found between the normal Wistar rats and SHR rats. Specific binding of 3H quinuclidinyl benzilate (QNB) increased and saturation was reached by 2 hours after incubation with slide- mounted brain tissue. The distribution of mAChR was heterogeneous along the fields of brain. Affinity (Kd) of mAChR was not different significantly among cortex, hippocampus and caudate-putamen. No difference was found between normal rats and SHR strain. More receptors (Bmax) were found in the cortex and hippocampus than in the caudate-putamen in normal rats. More receptors were found in the cortex and caudate- putamen in SHR rats than in normal rats. Radioreceptor assay and digital autoradiographic analysis of affinity and number of mAChR gave the same results. With the above findings, we concluded that we could use digital autoradiographic system with 3H-QNB in the characterization of mAChR of rats and that the cortex and caudate-putamen of SHR strain rats have more receptors than those of normal rats.