The development of RIA techniques for the measurement of urinary eicosanoid metabolites has allowed characterization of their biosynthesis and metabolism in health and disease. PGI2, TXA2, and LTC4 share a very low rate of secretion into the human circulation and rapidly disappear from circulation because of extensive enzymatic degradation. Long-term changes as well as episodic increases in their biosynthesis can be detected in association with various disease states, as reflected by increased excretion of major enzymatic metabolites such as 2,3-dinor-6-keto-PGF1 alpha, 11-dehydro-TXB2, and LTE4. Application of the same investigative approach to other recently identified products of arachidonate metabolism (2) may provide further insight into the pathophysiologic significance of this complex mediator system.