Thromboxane (Tx) A 2 is a platelet agonist, smooth muscle cell constrictor, and mitogen 1. Urinary Tx metabolite (Tx-M) excretion is increased in syndromes of platelet activation and early in both normal pregnancies and in pregnancy-induced hypertension 2, 3. A further increment occurs in patients presenting with severe preeclampsia, in whom Tx-M correlates with other indices of disease severity 4. TxA 2 exerts its effects through a membrane receptor (TP), of which two isoforms (α and β; refs. 5, 6) have been cloned. Overexpression of TP in the vasculature under the control of the pre-proendothelin-1 promoter 7 results in a murine model of intrauterine growth retardation (IUGR), which is rescued by timed suppression of Tx synthesis with indomethacin. IUGR is commonly associated with maternal diabetes or cigarette smoking, both conditions associated with increased TxA 2 biosynthesis 8, 9.