This study investigated the interaction between L-type Ca²⁺ currents (ICaL) and Ca²⁺ release from the sarcoplasmic reticulum (SRCR) in whole-cell voltage-clamped guinea-pig ventricular myocytes. Quasi-physiological cation solutions (Na⁺_o:K⁺_i)were used for most experiments. In control conditions there was no obvious interaction between ICaL and SRCR. In isoproterenol, activation of ICaL from voltages between -70 and -50 mV reduced the amplitude and accelerated the decay of the current. Short (50 ms), small amplitude voltage steps applied 60 ms or 510 ms before stimulating ICaL inhibited or facilitated the current, respectively. These changes were blocked by ryanodine. Low voltage activated currents such as T-type Ca²⁺ current, ICaTTX or "slip mode" Ca²⁺ conductance via INa⁺ were not responsible for low voltage SRCR. However, L-type Ca²⁺ currents could be distinguished at voltages as negative as -45 mV. It is concluded that in the presence of isoproterenol, Ca²⁺ release from the sarcoplasmic reticulum is due to activation of L-type Ca²⁺ channels.