P rostaglandins are present in large quantities in the kidney and have been shown to directly affect transepithelial transport. The present studies were designed to examine whether prostaglandin E2 could affect chloride transport across the thick ascending limb of Henle. Isolated segments of the cortical and medullary thick ascending limb of Henle were perfused in vitro and the transepithelial voltage and net chloride flux were measured. Exposure of the medullary thick ascending limb to 2 microM prostaglandin E2 resulted in a fall in net chloride transport of 40--50% with a concomitant fall in voltage. In contrast, net chloride transport in the cortical thick ascending limb was not affected by prostaglandin E2. Under similar conditions, the medullary thick ascending limb possessed twice the capacity to transport chloride than did the cortical thick ascending limb. The results suggest that endogenous renal prostaglandins may play a modulating role in the addition of salt to the renal medullary interstitium and may, under some circumstances, by chloruretic.