The effects of various tannins and related compounds on the actions of adrenocorticotropic hormone (ACTH) and insulin in fat cells isolated from rats were investigated. Hydrolyzable tannins such as geraniin, mallotusinic acid, chebulinic acid and chebulagic acid enhanced the ACTH-induced lipolysis at the concentration of 20 μg/ml or 5 μg/ml. On the other hand, these hydrolyzable tannins had no effect on the insulin-stimulated lipogenesis from glucose. Condensed tannins such as Ss-tannin 1 and RSF-tannin H showed weak inhibitory effects on the ACTH-induced lipolysis, while they enhanced the insulin-stimulated lipogenesis from glucose. Based on these results, the relationship between the structures and physiological actions of these tannins is discussed.