The effect of adenosine triphosphate (ATP) and its analogues was studied in longitudinal muscle strips of the rat gastric fundus in order to characterize the purinoceptors involved. At resting tension, 10(-4) M ATP usually induced a small initial relaxation followed by a contraction; when tone was raised by administration of carbachol (10(-7) M), ATP (10(-4) M) induced a larger relaxation followed by a smaller rebound contraction. Both the contraction at resting tension and the rebound contraction were antagonized by indomethacin. With raised tone, both ATP and 2-methylthioATP induced concentration-dependent relaxations, followed by small rebound contractions, but the slope of the concentration-response curve was very shallow. alpha, beta-MethyleneATP and adenosine induced only concentration-dependent relaxations and the maximal effect was much more pronounced than that of ATP and 2-methyl-thioATP. The rank order of potency of the purines producing relaxation was 2-methylthioATP greater than alpha, beta-methyleneATP greater than ATP greater than adenosine. The relaxant effect of ATP (10(-4) M) at raised tone was clearly antagonized by both reactive blue 2 (10(-4) M) and desensitization to alpha, beta-methyleneATP. It is concluded that the contractile effect of ATP in the rat gastric fundus is due to stimulation of prostaglandin biosynthesis, but identification of the purinoceptor subtype mediating relaxation is problematic and it may differ from the P2x- and P2y-receptors, which are clearly distinguishable in a number of other tissues.