The two cyclooxygenase (COX) enzymes catalyze the oxygenation of arachidonic acid to prostaglandin endoperoxides, which are the common intermediates in the biosynthesis of the bioactive lipids prostaglandins and thromboxane. COX-1 and COX-2 are approximately 60% identical in amino acid sequence, exhibit highly homologous three-dimensional structures, and appear functionally similar at the biochemical level. Recent work has uncovered a subtle functional difference between the two enzymes, namely the ability of COX-2 to efficiently utilize neutral derivatives (esters and amides) of arachidonic acid as substrates. Foremost among these neutral substrates are the endocannabinoids 2-arachidonoylglycerol and arachidonoylethanolamide. This raises the possibility that COX-2 oxygenation plays a role in a novel signaling pathway dependent on agonist-induced release of endocannabinoids and their selective oxygenation by COX-2. Among the products of COX-2 oxygenation of endocannabinoids are glyceryl prostaglandins, some of which (e.g. glyceryl prostaglandin E(2) and glyceryl prostaglandin I(2)) exhibit interesting biological activities in inflammatory, neurological, and vascular systems. These compounds are produced in intact cells stimulated with physiological agonists and have been isolated from in vivo sources. Important concepts relevant to the hypothesis of a COX-2-selective signaling pathway are presented.