Abstract: The effect of naphthylmethyl isoquinoline on amplitude of contraction and automaticity of guinea pig atria were investigated. The compound was found to markedly inhibit the force of contraction and automaticity in a concentration-dependent way. Propranolol competitively antagonized the effect of isoproterenol with-pA₂ of 7.5. Naphthylmethyl isoquinoline and verapamil antagonized isoproterenol, but in a noncompetitive manner with the pD₂' of 5.4 and 5.8, respectively.The effect of the compound on action potential and contractile force of guinea pig papillary muscles were also studied. It was shown to produce negative inotropic effects and shorten the fast action potential duration, but the maximal upstroke(Ⅴ_max) and amplitude of action potential were not affected. Thus, an excitation-contraction uncoupling was observed. It also depressed the amplitude. Ⅴ^max and duration of Ca²⁺-mediated slow action potential induced by high K⁺. Elevation of the CaCl₂ concentration from 2 to 5 mmol/L reversed its inhibition partially.These results indicate that naphthylmethyl isoquinoline has a blocking effect on calcium channels.