In a study of the effects of cobra cardiotoxins on myocardial tissue, both right ventricular (papillary muscle) and left ventricular contractility (isovolumic recording of left ventricular pulse pressure during Langendorff perfusion of the heart) were assessed. In papillary muscle, both toxins, at a concentration of 5 μg/mL, caused short-term increases in contractility to 200±25% and 171±15% for CTX-1 and CTX-2, respectively, at the point of maximum effect. At the same time, for CTX-1 and CTX-2, the time to peak tension (TPT) increased from 104±2 to 111±2 and from 96±2 to 104±5 ms, the relaxation time to 50%(TR50%) from 64±4 to 70±6 and from 64±6 to 69±7 ms, and the relaxation time to 95%(TR95%) from 163±10 to 190±22 and from 148±16 to 155±20 ms, respectively. This significantly differs from the positive inotropic effect of the β-adrenomimetic isoproterenol (170±31%), which causes acceleration of TPT from 106±5 to 89±4 ms, TR50% from 58±6 to 43±4 ms, and TR95% from 145±15 to 90±14 ms. When the whole heart was exposed to cardiotoxins, an increase in contractility was also observed, followed by its suppression and contracture, in contrast to isoproterenol, which caused a steady increase in contractility coupled with an increase in heart rate. Pretreatment of papillary muscles with the β-blocker propranolol (10 μM) did not prevent the development of cardiotoxin effects, but completely blocked the effects of isoproterenol. Our data indicate that the temporary increase in contractility under the action of cardiotoxins is not associated with the release of endogenous adrenaline, but rather is caused by changes in calcium homeostasis in cardiomyocytes.