The effects of ATP on the pancreatic vascular bed of the rat were studied under resting tone. ATP exerted two different effects depending on the concentration used: a slight vasodilatation in the 1.65-49.5 microM range which was statistically significant only at 16.5 microM and a concentration-related vasoconstriction in the 495-4 950 microM range. Theophylline, a P1 purinoceptor antagonist, did not modify the vasodilator effect of ATP. The existence of two P2 purinoceptor subtypes (P2y and P2x) in our preparation may be responsible for the dual effect of ATP. The P2y antagonist 2,2'pyridylisatogen (PIT) used at 5 microM, revealed a vasoconstrictor effect of ATP 165 microM, a concentration without effect per se. Furthermore, the transient vasoconstrictor effect of ATP 495 microM was changed into a long-lasting one in the presence of PIT. On the other hand, the blockade of P2x purinoceptors by the desensitizing agent, alpha,beta-methylene ATP, increased the vasodilator effect of ATP 16.5 microM. In conclusion, two subtypes of P2 purinoceptor do exist on the pancreatic vascular bed: P2y inducing vasodilatation and P2x inducing vasoconstriction. At vascular resting tone, the effect observed with ATP therefore depends on the concentration used and on the balance between P2y/P2x purinoceptors.