The double sucrose-gap method was used to examine the electrical responses of the guinea-pig vas deferens to ATP and their possible dependence on external Ca2+. Normally ATP induced a depolarization and an increase in membrane conductance and both effects were concentration-dependent. The reversal potential of the 10(-4) M ATP-induced depolarization was 27.1 mV positive to the resting membrane potential of the tissue. This value was quite similar to that previously obtained for the 3 X 10(-5) M ATP-induced depolarization. The smooth muscle membrane was depolarized by 5.9 mV in a Ca-free medium, in which ATP also caused a depolarization, associated with an increase in membrane conductance. The reversal potential of the depolarization induced by ATP (10(-4) M) in the Ca-free medium was 26.5 mV positive to the resting membrane potential. The results suggest that, in this tissue, ATP induces membrane depolarization with little effect on Ca2+ conductance.