The positive inotropic effect of anthopleurin-A (AP-A) was studied in vitro on isolated cat heart papillary muscles and in vivo in anesthetized and conscious dogs. In vitro, in low Ca2+ solution (1.27 mM), AP-A increased the force of contractions of isolated cat heart papillary muscles at concentrations from 0.2 × 10-8 M and higher; on a molar basis, AP-A was more than 200 times as potent as digoxin and on a weight basis, 33 times as potent. In vivo in anesthetized dogs, AP-A at 0.2 μg/kg/min i.v. increased myocardial contractile force; the geometric mean dose of AP-A required to increase the contractile force by 25% was 2.6 μg/kg; the corresponding dose of digoxin (infused at 2.8 μg/kg/min) was 107.4 μg/kg. The geometric mean lethal dose of AP-A for 8 dogs was 19.3 and that of digoxin 263.2 μg/kg i.v. The therapeutic index of AP-A was significantly higher than that of digoxin. All animals that received either AP-A or digoxin died in ventricular fibrillation. The reversal of t-wave was typical for AP-A. As measured by left ventricular pressure telemetry. AP-A, 2 μg/kg i.v. single dose, increased LV dp/dt max in conscious dogs for longer than 2 hr.