A recording microspectro-photometer was adapted for continuous measurement of chlorphenol red (CPR) accumulation in luminal fluid of single flounder tubules bathed with oxygenated medium containing low concentrations of this organic anion. Steady-state data are consistent with published work based on visual methods. Kinetic data provide the first detailed evaluation of separate influx and efflux processes. CPR influx across tubular cells exhibited the classical features of active, uphill transport. Two key characteristics of CPR efflux, common to organic anion efflux from a number of vertebrate renal tissue preparations, were downhill movement proportional to luminal CPR concentration with no evidence of competition from the luminal side and biphasic action of competitor anions from the medium side, i.e., enhancement of CPR efflux by low-and depression by high-competitor levels in medium. Although such findings suggest countertransport of organic anions across tubular cells, three experimental tests with CPR all gave negative results and further work is needed to determine whether characteristics of anion efflux derive from intricacies of multiple transport steps or a single, heretofore unrecognized, membrane phenomenon.