1. The effect of suramin on the concentration-effect curve for the contractile response of the isolated mouse vas deferens to alpha, beta-methylene ATP (alpha, beta-meATP) was investigated. 2. The concentration-response curve to alpha, beta-meATP had a consistent discontinuity at about 3 x 10(-6) M, giving it a biphasic appearance. 3. Suramin in a dose-dependent, reversible manner both shifted the curve to the right and at the same time elevated the maximum response. 4. The P2y inhibitor Reactive Blue 2, on the other hand, both shifted the curve to the left and elevated the maximum response. 5. These results show that alpha, beta-meATP in this preparation is an agonist both at excitatory, presumably P2y receptors and inhibitory P2y receptors, and that suramin antagonizes both effects.