We investigated the effects of breeding conditions on neurochemical markers, muscarinic receptor (mAChR), beta-adrenoreceptor (beta-AdrR), imipramine binding sites (IMBS), choline acetyltransferase (ChAT), acetylcholinesterase (AChE) and monoamine oxidase (MAO) activity in aged rat brains. An increase of affinity (Kd) and the decrease in the number (Vmax) of mAChR were found in the individual aged rats. Concerning IMBS, Kd and Vmax values increased in the individual aged rats. However, no significant changes were observed in the beta-AdrR. The increases of ChAT and MAO activity were found in the aggregated aged rats to compare with in the individual aged rats, while AChE activity decreased in the aggregated aged rats. These changes were also particularly seen in the forebrain of aged rats. These results indicate that the functions of the central nervous system may be reduced in the individual aged rats to compare with in the aggregated aged rats under the breeding conditions.