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Thromboxane A2 (TXA2) is a potent platelet aggregating agent and constrictor of coronary, vascular, and bronchial smooth muscle. This Thesis is concerned with the actions on human platelets of 5 endoperoxide analogues, EP035, EP037, EP043, EP045 and EP092. These analogues show competitive receptor blockade of TXA2 action on smooth muscle: it was of particular interest to determine whether additional modes of action contributed to their anti-aggregatory effects in platelets. The purification of a binding protein from sheep muscle and the development of a protein binding assay for the measurement of platelet cAMP levels are described. EP035 increases platelet cAMP levels markedly; this is thought to be due to a partial agonist effect on the PGI2 receptor of human platelets. EP092 is the only other analogue to raise basal cAMP levels. Although this effect is very weak, it may be sufficient to augment the action of EP092 when high con¬ centrations are used. The development of a GC-MS assay for the measurement of platelet TXB2 levels is described. EP043 was found to inhibit the biosynthesis of TXB2 from arachidonic acid (AA) added exogenously to platelets. EP092 shows a slight inhibitory effect but at ten-fold higher concentra¬ tions. The inhibition of AA metabolism by EP035 is suggested to result from its action to increase platelet cAMP levels. The development of a binding assay for 3H 9,11-epoxymethano PGH2 to whole platelets is described. Total binding appears to be the sum of binding to a non-specific saturable site, uptake of the lipo¬ philic ligand and binding to a stereospecific, saturable site. The specific saturable component of binding can be displaced by TXA2