The effects of garlic juice on smooth and cardiac muscles of rabbit and guinea pig were tested in vitro using isolated segments of aorta, trachea and intestines and isolated rabbit hearts. Garlic juice inhibited the contractions of rabbit and guinea pig aortic rings induced by norepinephrine in Ca(2+)-free and Ca(2+)-containing Krebs-Henseleit solutions. Also, garlic juice inhibited the contractions of rabbit and guinea pig tracheal smooth muscles induced by acetylcholine and histamine, respectively, in both Ca(2+)-free and Ca(2+)-containing Krebs-Henseleit solutions. Furthermore, garlic juice inhibited the spontaneous movements of rabbit jejunum and guinea pig ileum and inhibited the force of contraction of isolated rabbit hearts in a concentration-dependent manner. All inhibitions were reversible. These data suggest that the hypotensive action of garlic juice may be due, at least in part, to a direct relaxant effect on smooth muscles.