Canine cardiac Purkinje cells contain both L- and T-type calcium currents, yet the single Ca2+channels have not been characterized from these cells. Additionally, previous studies have shown an overlap between the steady-state inactivation and activation curves for L-type Ca2+currents, suggesting the presence of L-type Ca2+“window” current. We used the on-cell, patch-clamp technique to study Ca2+channels from isolated cardiac Purkinje cells. Patches contained one or more Ca2+channels 75% of the time. L-type channels were seen in 69% and T-type channels in 73% of these patches. With 110 mM Ba2+as the charge carrier, the conductances of the L- and T-type Ca2+channels were 24.2±0.8 pS (n=9) and 9.0±0.5 pS (n=8), respectively (mean±SEM). With 110 mM Ca2+as the charge carrier, the conductance of the L-type Ca2+channel decreased to 9.7±1.2 pS (n=4), whereas the T-type Ca2+channel conductance was unchanged. Voltage-dependent inactivation was shown for both L- and T-type Ca2+channels, although for L-type Ca2+channel with Ba2+as the charge carrier, inactivation took at least 30 seconds at a potential of +40 mV. After channel inactivation was complete, L-type Ca2+channel reopenings were observed following repolarizing steps into the window voltage range. Thus, our data identify both L- and T-type Ca2+channels in cardiac Purkinje cells and demonstrate, at the single-channel level, L-type channel transitions expected for a window current. Window current may play an important role in shaping the action potential and in arrhythmogenesis. (Circulation Research1992;70:456–464)