The specific Na channel blocker tetrodotoxin (TTX) produces a direct negative inotropic effect on the isolated guinea-pig papillary muscle stimulated at 1 Hz. The toxin is significantly more potent in reducing maximal upstroke velocity of the transmembrane action potential (Vmax, an index of the available Na conductance) than in reducing force of contraction (Fc), the IC50 for Vmax divided by the IC50 for Fc being 0.23 (95% confidence interval, 0.16-0.43). This IC50 ratio defines the negative inotropic influence of Na channel blockade per se, because TTX has no known action other than blocking Na channels. Vmax-reducing to negative inotropic concentration ratios were also obtained for 5 widely used and 2 experimental antiarrhythmic agents belonging to the Na channel-blocking class (class 1). Five of these drugs (aprindine, CCI 22277, disopyramide, mexiletine, and quinidine) differed significantly from TTX in that the Na channel block was associated with a stronger negative inotropic effect. It is concluded that one or more mechanisms in addition to Na channel blockade are involved in the negative inotropic effect of these antiarrhythmic drugs. If the antiarrhythmic activity of class 1 agents depended solely on Na channel block, more selective agents with less negative inotropic action could conceivably be developed.