Recently we have demonstrated the presence of significant amounts of prostaglandins in venous blood from normal human subjects (1). Thin layer chromatography and parallel bioassay using several different isolated organ preparations established that the extracted prostaglandin-like activity consisted of a mixture of prostaglandins E 1, E 2, F 1α and F 2α. The possibility arose that determinations of prostaglandin activity in whole human blood might be useful in the study of disease states involving increased prostaglandin formation or release. In order to increase the reproducibility of values obtained we have investigated the effect of venous occlusion, fasting and exercise on whole blood prostaglandin concentrations in healthy human subjects.