There is an autocrine relationship between eicosanoid and cytokine synthesis, with the ratio of prostaglandin E 2 (PGE 2)/thromboxane A 2 (TXA 2) being one of the determinants of the level of cytokine synthesis. In monocytes, cyclooxygenase type 1 (COX-1) activity appears to favor TXA 2 production and COX-2 activity appears to favor PGE 2 production. This has led to speculation regarding possible linkage of COX isozymes with PGE and TXA synthase. We have studied the kinetics of PGE 2 and TXA 2 synthesis under conditions that rely on COX-1 or-2 activity. With small amounts of endogenously generated prostaglandin H 2 (PGH 2), TXA 2 synthesis was greater than PGE 2. With greater amounts of endogenously generated PGH 2, PGE 2 synthesis was greater than TXA 2. Also, TXA synthase was saturated at lower substrate concentrations than PGE synthase. This pattern was observed irrespective of whether PGH 2 was produced by COX-1 or COX-2 or whether it was added directly. Furthermore, the inhibition of eicosanoid production by the action of nonsteroidal anti-inflammatory drugs or by the prevention of COX-2 induction with the p38 mitogen-activated protein kinase inhibitor SKF86002 was greater for PGE 2 than for TXA 2. It is proposed that different kinetics of PGE synthase and TXA synthase account for the patterns of production of these eicosanoids in monocytes under a variety of experimental conditions. These properties provide an alternative explanation to notional linkage or compartmentalization of COX-1 or-2 with the respective terminal synthases and that therapeutically induced changes in eicosanoid ratios toward predominance of TXA 2 may have unwanted effects in long-term anti-inflammatory and anti-arthritic therapy.