In the course of investigations of gamma-oryzanol in rats, circadian rhythms in antiulcerogenic action on gamma-oryzanol were observed. Circadian rhythms in several other parameters were investigated and correlated with the antiulcerogenic action. Pretreatment of rats with gamma-oryzanol 100 mg/kg, s.c., for 5 days tended to decrease serum gastrin levels and gastric secretion, and to increase serum 11-OHCS. Suppresive patterns of serum gastrin and gastric secretion following the pretreatment with gamma-oryzanol corresponded to the circadian rhythms in antiulcerogenic action of gamma-oryzanol. A complication of our results suggests that increment of catecholamines in rat brain after gamma-oryzanol plays a role in the above mentioned actions.