Extracts of the essential oil of Croton zehntneri (CzEO) yield a mixture of substances in which the major components are anethole and estragole. The action of CzEO and of pure anethole and estragole (at 0.05-1.0 mg/ml) were studied in muscle preparations of the toad and rat. All three oils blocked twitches evoked by nerve stimulation. In the rat diaphragm direct stimulation resulted in enhanced twitches (CzEO), partially depressed twitches (estragole) and anethole did not induce a significant change. The response of toad rectus to acetylcholine was reduced by all three oils. Caffeine contractions of the rectus abdominis and sartorius muscles were enhanced by all oils. This enhancement in the case of the sartorius was blocked by procaine. Lowering the temperature to 4 degrees C in the presence of CzEO, anethole or estragole provoked a contraction which could be blocked by procaine. CzEO, anethole or estragole alone sometimes caused contractions. The occurrence of this response was increased by elevation of calcium (to 22 mM). Other divalent cations and La3+ were also effective, in order: La3+ > Ca2+ > Mn2+ > Ba2+ > Mg2+ except in the case of anethole where the order of Ca2+ and Mn2+ are interchanged. CzEO- and estragole-induced contractions were blocked by procaine. The data suggest that CzEO, anethole and estragole may have two sites of action on muscle fibers: the post junctional membrane, by blocking neuromuscular transmission, and on the sarcoplasmic reticulum (SR), by increasing myoplasmic calcium.