Several unesterified prostaglandin derivates are produced by sheep vesicular gland homogenates ipz* &tro1-3. Prolonged synthesis of these derivates ilt viva would tend to deplete the available substrates and draw upon precursors of the free fatty acids since the level of unesterified fatty acids is generally low in tissues. As one possibility, the large reservoir of polyunsaturated acids bound in ester form might undergo oxidative cyclization to prostaglandin either prior to or following hydrolysis. To investigate whether the cyclization reaction could occur directly with the phospholipids, I-palmitoyl-z-([z’-14C] eicosatrienoyl) gfycero-3-phosphorylcholine was prepared and incubated with enzyme preparations from sheep vesicular glands. Although unesterified prostaglandins were formed from the phospholipid, no appreciable amount of radioactive prostaglandin was found esterified to phospholipid. These results sug-