We investigated the relationship between voltage-operated Ca²⁺ channel current and the corresponding intracellular Ca²⁺ concentration ([Ca²⁺]_i) change (Ca²⁺ transient) in guinea pig gastric myocytes. Fluorescence microspectroscopy was combined with conventional whole cell patch-clamp technique, and fura 2 (80 µM) was added to CsCl-rich pipette solution. Step depolarization to 0 mV induced inward Ca²⁺ current (I_Ca) and concomitantly raised [Ca²⁺]_i. Both responses were suppressed by nicardipine, an L-type Ca²⁺ channel blocker, and the voltage dependence of Ca²⁺ transient was similar to the current-voltage relation of I_Ca. When pulse duration was increased by up to 900 ms, peak Ca²⁺ transient increased and reached a steady state when stimulation was for longer. The calculated fast Ca²⁺ buffering capacity (B value), determined as the ratio of the time integral of I_Ca divided by the amplitude of Ca²⁺ transient, was not significantly increased after depletion of Ca²⁺ stores by the cyclic application of caffeine (10 mM) in the presence of ryanodine (4 µM). The addition of cyclopiazonic acid (CPA, 10 µM), a sarco(endo)plasmic reticulum Ca²⁺-ATPase inhibitor, decreased B value by ~20% in a reversible manner. When KCl pipette solution was used, Ca²⁺-activated K⁺ current [I_K(Ca)] was also recorded during step depolarization. CPA sensitively suppressed the initial peak and oscillations of I_K(Ca) with irregular effects on Ca²⁺ transients. The above results suggest that, in guinea pig gastric myocyte, Ca²⁺ transient is tightly coupled to I_Ca during depolarization, and global [Ca²⁺]_i is not significantly affected by Ca²⁺-induced Ca²⁺ release from sarcoplasmic reticulum during depolarization.