Fluorides were supposed to exert a stimulatory action on the catecholamine release. In this study, the authors attempted to investigate the action of sodium fluoride on the catecholamine release from the isolated perfused cow adrenal gland and rat heart. And also the inhibitory effect of sodium fluoride on the monoamine oxidase activity in rat heart and liver mitochondria was investigated. The monoamine oxidase activity was measured by the conversion of benzylamine to benzaldehyde. The results obtained were follows; 1. Sodium fluoride stimulated the release of catecholamine from the isolated perfused cow adrenal gland and rat heart. 2. Sodium fluoride inhibited the rat heart and liver mitochondrial monoamine oxidase activity.