The changes in density of M-cholinergic receptors in different areas of senile rats and the regulatory action of Huang Qi (黄芪Radix Astragali, a drug for warming yang and replenishing qi) were observed by autoradiography. The results showed that the gray scale displayed in brain sections was clear and mainly distributed in the cortex, hippocampus and striate body, while that due to nonspecific combination was negligible. The gray scale in the cortex, hippocampus and striate body of the experimental group was markedly lower than that in the young control rats, decreased respectively by 24.87%, 14.12% and 12.76% (all P<0.05); but it was obviously higher than those in the senile control rats, increased respectively by 24.15%, 14.38% and 13.47% (P<0.05). The data indicate that Huang Qi (黄芪Radix Astragali) may up-regulate the decreased density of M-cholinergic receptors in the brain of senile rats.