The pithed guinea pig is introduced as a model for evaluating positive inotropic and vasodilator effects of substances. The model was characterized by means of ouabain, isoprenaline and hydralazine. In this model, the effects of some new noncatecholamine, nonglycoside positive inotropic agents were compared with respect to an increase in LV-dp/dtmax, effects on heart rate and vasodilator effects. Comparing equieffective doses (inducing an increase in LV-dp/dtmax by 1,000 mm Hg/s), the rank order of potency was AR-L 100 > AR-L 57 > MDL-17043 > AR-L 115 > amrinone. The investigated substances also exhibited vasodilating effects. Comparing the doses which decreased diastolic blood pressure by 25 mm Hg, the order of potency was AR-L 57 > AR-L 115 > MDL-17043 > amrinone > AR-L 100.