We studied post-occlusive reactive hyperaemia using ecg-triggered mercury strain-gauge plethysmography in eight normal subjects treated with incremental doses of aspirin (27.5-1200 mg). The reactive hyperaemic response was measured in the finger (predominantly skin blood flow) and the calf (predominantly muscle). Concentrations of TXB2 and 6-oxo-PGF1 alpha were measured in venous effluent blood from the hand by RIA, following arterial occlusion. Levels of TXB2 were significantly higher at 0-10 and 60-70 seconds (p less than 0.01), and 90-100 seconds (p less than 0.05) following release of occlusion compared to pre-occlusion values. However there was no significant change in concentrations of 6-oxo-PGF1 alpha and therefore by this method release of prostacyclin during reactive hyperaemia in the hand. Aspirin had no influence on finger or calf reactive hyperaemia 90 minutes after dosing, despite marked inhibition of platelet MDA production (75% after 110 mg, maximal inhibition after 1200 mg). These data provide no support for the hypothesis that prostacyclin is involved in the determination of the post-occlusive reactive hyperaemic response in the finger and calf in man.