1 The actions of the prostaglandin H2 (PGH2) analogue, U-46619, have been compared with those of PGH2 on thromboxane A2 (TxA2) on a range of isolated smooth muscle preparations in a superfusion cascade system. 2 U-46619 was a potent agonist on guinea-pig lung strip, dog saphenous vein and rat and rabbit aortae. In contrast, U-46619 was weak or inactive on guinea-pig ileum and fundic strip, cat trachea and dog and cat iris sphincter muscles, preparations on which either PGE2 or PGF2 alpha was the most potent agonist studied. 3 PGH2 was active on all of the preparations and displayed little selectivity. On some of the preparations, the actions of PGH2 may have been mediated indirectly by conversion to other prostanoids. 4 In contrast, TxA2 displayed the same pattern of selectivity as U-46619, being a potent agonist on the lung strip and vascular preparations but weak or inactive on the others. 5 It is suggested that U-46619 is a selective TxA2-mimetic and that it should therefore be a valuable tool in the study of the actions of TxA2.