Furegrelate sodium (U-63,557A), a pyridine-derivative thromboxane synthase inhibitor, was administered orally in single doses of 200 to 1600 mg to normal male subjects. Furegrelate produced a dose-related inhibition of thromboxane synthesis for 8–12 hours when measured either ex vivo from platelet-rich plasma (PRP) or in vivo from urine. In general, the extent of thromboxane synthesis inhibition was greater in PRP than in urine. Furegrelate significantly inhibited platelet aggregation, but the effect was variable and measurements of thromboxane synthase did not predict the impact on platelet aggregation. Bleeding times and coagulation parameters were not altered significantly. Furegrelate was well absorbed orally with Tmax = 1 hr and t½ = 3.5 to 5 hrs. There was no marked metabolism; elimination was primarily by renal excretion of parent compound. Thus, furegrelate is an effective inhibitor of thromboxane synthase in man with a relatively long biologic and circulating half-life.