A series of trichloroacetamidine derivatives, obtained by addition of amines to trichloroacetonitrile, was evaluated for positive inotropic activity on isolated cat heart papillary muscles. Increased contractility, not antagonized by beta-adrenergic blockade with sotalol or reserpine pretreatment, was observed in this assay with a variety of N-substituted trichloroacetamidine derivatives. More extensive pharmacological studies with the 3-indolylmethyl analogue 2 showed that this amidine in dogs, 5 mg/kg iv, produced a positive inotropic effect more pronounced than that of ouabain, 50 microgram/kg iv. Several of the trichloroacetamidines were found to be inhibitors of guinea pig kidney and calf heart Na-K-dependent ATPase and to have specificity for these enzymes different from that of ouabain. Bacterial mutagenic activity was observed with three members, 2,3, and 12, of the series.