The causal role assigned to the E and F prostaglandins in inflammatory processes, implied by the antiinflammatory action of prostaglandin synthetase inhibitors, is not consistent with the findings reported here that a compound (MK-447) capable of increasing levels of these prostaglandins is antiinflammatory in classical animal models of acute inflammation. That both MK-447 and prostaglandin synthetase inhibitors depress the enzymatic formation of PGG2 from arachidonic acid suggests that this endoperoxide plays a pivotal role in acute inflammation. However, in view of the intermediate nature of PGG2, it seems likely that such a pivotal role for this substance is a function of its ability to be converted to other inflammatory mediators. Possible candidates for a causal role are thromboxane A2 (TXA2) prostacyclin (PGI2), both of which derive from PGG2. However, direct evidence is presented to show that an oxygen equivalent released in the enzymatic conversion of PGG2 to PGH2 is a prime factor in inflammation.