Renomedullary prostaglandin synthesis is lower in salt-sensitive (S) Dahl rats than in age-matched salt-resistant (R) rats on either 0.6% or 8.0% NaCl. The possible role of substrate availability to the cyclooxygenase in determining these differences was examined. S and R rats were started on diets with variable salt content (0.6%, 4.0% or 8.0% NaCl) at five weeks and were sacrified at either 11 or 16 weeks of age for comparisons of renomedullary phosphollpase A1and A2activities (EC 3.1.1.4. and 3.1.1.32) and arachidonate incorporation into the lipids of renal medulla. Both phospholipase activities were higher in R rats at the two ages examined and for all levels of salt intake. These differences could not be accounted for by variable amounts of endogenous phospholipid substrate. High salt intake did not influence the in vitro A2deacylating activities per mg protein. Arachidonate incorporation into renomedullary triglycerides remained constant with age in both strains while phospholipid labeling declined with age in S rats only so that, at 16 weeks of age, S rats had significantly lower incorporation into membrane phospholipids. This age-related decline in phospholipid labeling in S rats was prevented by 4.0% NaCl. These results suggest that low renomedullary phospholipase activities in S rats may restrict the release of substrate for prostaglandin synthesis and may also be related to reduced rates of fatty acid incorporation into membrane phospholipids.