To study the significance of placental thromboxane A2 and prostacyclin in the regulation of placental blood flow, intervillous blood flow was measured using a 133Xenon method from 39 women zero to two days before delivery and compared it with the placental production of thromboxane A2 and prostacyclin as measured with a superfusion method postpartum. The placental production of thromboxane B2 (a metabolite of thromboxane A2; 4.5 +/- 1.3 ng/minute per gram dry weight of tissue; mean +/- SD) and that of 6-keto-prostaglandin F1 alpha (a metabolite of prostacyclin; 0.64 +/- 0.27 ng/minute per gram) did not correlate significantly with intervillous blood flow (153.1 +/- 108.0 mL/minute per 100 mL; r = -0.308 and 0.245, respectively), whereas the thromboxane B2/6-keto-prostaglandin F1 alpha ratio (8.53 +/- 4.3) was inversely related to intervillous blood flow (r = -0.419; P less than .01). In the women with intervillous blood flow below the normal mean (less than 130 mL/minute per 100 mL; N = 20) placental thromboxane B2 production (5.1 + 1.2 ng/minute per gram) was higher (P less than .005) and that of 6-keto-prostaglandin F1 alpha (0.54 +/- 0.23 ng/minute per gram) lower (P less than .02) than those in women with intervillous blood flow above 130 mL/minute per 100 mL (thromboxane B2 4.01 +/- 1.0 and 6-keto-prostaglandin F1 alpha 0.75 +/- 0.27 ng/minute per gram; N = 19). These results suggest that placental thromboxane A2 and prostacyclin may be factors in the regulation of intervillous blood flow and that their balance of production is more important than the presence of either agent alone.(ABSTRACT TRUNCATED AT 250 WORDS)