Abstract: Since 1972 gossypol, a biologically active constituent of cotton seed and cotton root bark, was shown to have a potent spermatocidal effect in man. Glinical trials pointed out that occasional cases of hypokalemia and hypokalemic paralysis might happen in the course of gossypol administration. Little was known concerning the intrinsic relationship between gossypol and such hypokalemias.The authors indicated that in isolated rabbit heart brief perfusion with Lock's solution containing gossypyl 2 mg/l could induce a significant loss of the potassium content of the left ventricular myocardium, together with a drastic depression of its contractility. These actions were reversed when Mg²⁺ (2～4 me/l) had been added beforehand to the perfusion solution. It is suggested that the potassium-depleting (and negative inotropic) effect of gossypol on the isolated heart might be the consequence of its possihle inhibitory action on Na-K-ATPase (and/or other Mg-dependent enzyme systems involved in energy metabolism), thus interfering in the active potassium transport.