Abstract The ionic mechanisms underlying concentration-related alterations in the action potential configuration caused by ATP were studied using preparations of the guinea-pig vas defercns voltage-clamped by a double sucrose gap method. Under current-clamp conditions, ATP at concentration of 1.6 μM enhanced the rates of rise and of repolarisation of the action potential whereas at concentration of 1.6 mM it reduced both rates. Under voltage-clamp conditions, lower concentrations increased the maximum inward Ca current without altering kinetics or reversal potential. Higher concentrations reduced the maximum inward Ca current with slowing of rates of activations and inactivation, but also caused a negative shift in reversal potential without affecting conductance. These results suggest that a low ATP concentration activates the voltage-dependent Ca current channels and that the action of a high ATP concentration is related to the internal Ca ion concentration.