1 The effects of SC-19220, sodium meclofenamate, indomethacin or trimethoquinol were studied on contractions of the rat stomach longitudinal muscle to prostaglandin D2 (PGD2), PGE2, PGF2 alpha, PGH2, epoxymethano PGH2 analogues, PGI2, 6-keto-PGF1 alpha, 6,15-diketo-PGF1 alpha and thromboxane B2. All the drugs reduced contractions to all the prostanoids, but the degree of reduction differed widely. Selectivity of blockade was assessed by comparison with acetylcholine (ACh). 2 With SC-19220 5 micrograms/ml the effect on thromboxane B2, PGD2 or PGH2 and its epoxymethano analogues was not significantly different from the small effect on ACh, but the other prostanoids were blocked to greater extents. 3 The effect of the cyclo-oxygenase inhibitor sodium meclofenamate, 1 or 2 micrograms/ml, on 6,15-diketo-PGF1 alpha or thromboxane B2 was similar to the small antagonism of ACh, whereas the other prostanoids were blocked to greater extents. Indomethacin, 1 microgram/ml, also reduced contractions to the prostanoids, but antagonism of the PGH2 epoxymethano analogues was considerably less than with meclofenamate. 4 The beta-adrenoceptor stimulant trimethoquinol, 50 ng/ml, was the most potent prostanoid antagonist tested; all the prostanoids except PGE2 were antagonized more than ACh.