The possibility of naturally occurring amnesia was examined in aging Fischer F254 rats. Animals 110 weeks old showed a loss of learning and memory in a passive avoidance responses (PAR) failure test. When 120 mg/kg/day of Shosaikoto extract was placed in the diet after 72 weeks of age to 110 weeks, it improved age-induced PAR failure in both the two-compartment avoidance box test and the step-down test. The responses of 110-week-old Shosaikoto-treated rats on both PAR failures were almost the same as those of non-treated 6-week-old rats. In these Shosaikoto-treated rats, dopamine was increased and norepinephrine and vanillyl mandelic acid were decreased in brain relative to those in non-treated aging rats.