Ethyl 1-benzyl-3-hydroxy-2(5H)-oxopyrrole-4-carboxylate (1, EBPC) is a potent and specific inhibitor of aldose reductase. It was greater than 4000X more potent in its inhibition of rat lens aldose reductase than the closely related rat or pig kidney aldehyde reductase, thus making it the most selective inhibitor of a NADPH-dependent carbonyl reductase identified to date. In agreement with this observation, it was found to be a highly potent inhibitor of aldose reductase from rat sciatic nerve with greater than 98% inhibition at 1 microM, but it was practically devoid of activity against aldehyde reductases from rat liver and brain. Inhibition of aldose reductase was mixed type for glyceraldehyde (Ki = 8.0 x 10(-8) M) and noncompetitive for NADPH (Ki = 1.70 x 10(-8) M). Its potential as an in vitro tool to quantitate monomeric aldo/keto reductase activities in crude tissue extracts is presented. Structure-activity relationships emerging from synthetic modifications of EBPC are discussed. Several modifications were found to be active in vitro against aldose reductase from human placenta and in vivo in a rat model of diabetic complications, but none was more potent than EBPC.