The racemate and the d-isomer of 6,7-dichloro-2,3-dihydro-5-(2-thienylcarbonyl)benzofuran-2-carboxylic acid exhibited diuretic activity in the chimpanzee, dog and rat. In the chimpanzee, the diuresis and natriuresis presumably resulted from a site of action in the thick ascending limb of Henle's loop. The l-isomer was uricosuric but devoid of any diuretic action in the chimpanzee and similarly was not diuretic in the dog and rat. Various proportions of the two isomers were tested in the chimpanzee in an attempt to produce an optimal diuretic and uricosuric profile. It was concluded that the racemic mixture, under these experimental conditions, was responsible for the most effective overall response. This is apparently the first time in which there is a distinct separation of diuretic and uricosuric actions in the enantiomers of a racemic diuretic uricosuric agent.