In guinea pigs, 160 mug kg-1 of ouabain decreased noradrenaline content of the right atrium and left ventricle by 44% and 68%, respectively, 2 hr after intraperitoneal injection. In rats, 20 mg kg-1 decreased noradrenaline content of the right atrium and left ventricle by 35% and 55%, respectively, at 1 hr. In mice, 10 mg kg-1 decreased myocardial noradrenaline by 33% and 20 mg kg-1 by 45% at 15 min. Phenytoin, 10 mg kg-1 in the guinea pig, 80 mg-1 in the rat, and 40 mg kg-1, in the mouse suppressed the effect of ouabain. Hexamethonium did not suppress the effect. In isolated, perfused guinea pig and rat hearts, 5.5 + 10-8 M and 6.8 + 10-6 M, respectively, caused an increase in noradrenaline output, which was usually followed by oscillatory fluxes; the initial increase in output was obtunded and the oscillations were entirely prevented by premedication with phenytoin but not by the addition of the drug to the perfusion medium.