We have recently developed 1-([3-O-methyl-11C]3,4-dimethoxyphenethyl)-4-(3-phenylpropyl) piperazine ([11C]SA4503) as a selective radioligand for mapping sigma1 receptors in the brain by positron emission tomography (PET). In the present short communication we evaluated the age-related changes of the binding of this ligand to sigma1 receptors in Fisher-344 rats (1.5-, 6-, 12-, and 24-month-old) by the in vitro binding assay. We also measured the binding of [3H](+)-pentazocine to sigma1 receptors and the binding of [3H]1,3-di-O-tolylguanidine to sigma2 receptors, which are current standard methods. The specific binding of the three radioligands increased age-dependently. Both Kd and Bmax values of the 24-month-old rats for each radioligand were significantly higher than those of the young rats (1.5- and 6-month-old). The increased numbers of both sigma1 and sigma2 receptor subtypes in the aged rats compensate for the lowered affinity, and rather enhanced the radioligand-receptor binding. The results contrast strikingly with the age-dependent decrease in the dopaminergic, cholinergic and glutamatergic receptors that are reported to be correlated with the sigma receptors, and indicate that a PET study with [11C]SA4503 to evaluate the aging process in humans would be of great interest.