Aging changes of somatostatin (SS) mRNA-positive neurons in the dorsal hippocampus of old rats were studied by in situ hybridization histochemistry and image analysis. In the young rats,SS mRNA-positive cell bodies were mainly located in the pyramidal and polymorphic cell layers of the CA1 and CA2 regions, and in the strata radiatum and polymorphic cell layer of the CA3 region, and in the polymorphic cell layer of dentate gyrus. In the old rats,SS mRNA-positive cell bodies were concentrated in the polymorphic cell layer and denate gyrus. A marked decrease of number of SS mRNA-positive cell bodies and a significant increase of gray value of SS mRNA-positive cell bodies were found in the dorsal hippocampus and dentate gyrus of the old rats as compared with the young rats. No significant age difference in the sectional area of SS mRNA-positive cell bodies was observed in the dorsal hippocampus. The results showed that there were apparent changes with aging of the SS mRNA-positive cell bodies in the dorsal hippocampus of the rats. The significance of the changes remains to be studied further.