The responsiveness of the cholinergic receptor (ChR) of the longitudinal muscle of guinea-pig isolated ileum was characterized by determining the ED50 of acetylcholine (ACh) and pA2 of mepenzolate (MPZ) in tissues from animals of different ages. The ages of guinea pig were 0.75, 3, 6 and 24 months respectively. According to our data, the sensitivity of ChR decreased with age peaking around 6 months after which time there was an increase in sensitivity. There was no difference in the pA2. The results of this investigation suggest that changes in the ChR with age may have potential in the understanding of aging process and the use of age rather than the weight of animal may provide more reproducible data in studies using isolated preparations.