Inotropic effects of non-depolarizing muscle relaxants were examined with guinea pig ventricular papillary muscle depolarized to -47 mV in high K Ba-Tyrode solution. Field stimulation of 0.1 Hz elicited the slow action potential, a measure of the calcium current. The amplitude, the duration at 0 mV level and dV/dt of the action potential were monitored together with the contractile tension. Amelizol (3 mg X ml-1 d-tubocurarine (d-tc) and 5 mg X ml-1 chlorobutanol) depressed the four functions in a dose-dependent manner, while crystalline d-tc did not. Chlorobutanol (the antimicrobial preservative) had the same effects as Amelizol. Neither Mioblock (2 mg X ml-1 pancuronium and unpublished preservative) nor crystalline pancuronium altered the functions. These findings suggest that the negative inotropic effect of Amelizol is not due to d-tc but to chlorobutanol, which may exert its effect by depressing the calcium current. The lack of change in the slow action potential seen with pancuronium may indicate no direct effect on the calcium current, thereby further suggesting absent direct beta-adrenomimetic action of this agent.