Sodium currents (INa) and T-type calcium currents (ICa,T) of isolated guinea-pig ventricular myocytes were recorded using the whole-cell voltage-clamp technique. Separation of the two currents was obtained by using the difference current method in the presence and absence of 2 mM extracellular Na (Nao). Time to peak and the time constant of inactivation of. INa were about 5 times faster than that of ICa,T (test potential -30 mV), and ICa,T had an activation range positive to -50 mV, were inactivated at -50 mV, and their current/voltage relationships peaked at -22.3 +/- 1.8 mV (n = 18) and -29.3 +/- 0.5 mV (n = 18) respectively, with a reversal potential of +40.3 +/- 4 mV (n = 18) and +30 +/- 10 mV (n = 18), respectively [2 mM Nao; 5.4 mM extracellular Ca (Cao)]. INa was blocked by 30 microM tetrodotoxin (TTX), 500 microM lidocaine, partly inhibited by 1 mM amiloride, but not affected by 100 microM nickel (Ni). ICa,T was neither affected by 30 microM TTX nor 500 microM lidocaine, but blocked by 100 microM Ni, 1 mM amiloride, 10 microM R 56865 and use-dependently reduced by 5 microM flunarizine. Adenosine (500 microM) affected neither INa nor ICa,T, whereas 1 microM isoprenaline did not affect ICa,T, but slightly increased INa. Our results demonstrate that the characteristics of ICa,T are not affected by the concomitant activation of INa, and vice versa. We conclude that ICa,T are not Ca currents through Na channels.