To characterize volume regulatory mechanisms, unidirectional Rb+ efflux and influx, unidirectional Cl- influx, and cell volume were measured in cultured human retinal pigment epithelium (HRPE). The HRPE was found to be capable of both regulatory volume increase (RVI), in response to a hypertonic challenge, and regulatory volume decrease (RVD), in response to a hypotonic challenge. Bumetanide-sensitive Rb+ influx increased almost threefold on incubation in a hypertonic (390 mosmol/kgH2O) medium. Bumetanide-insensitive Rb+ influx was activated by hypotonic (190 mosmol/kgH2O) challenge as well as by treatment with N-ethylmaleimide (NEM). Exposure to hypotonic media also activated unidirectional Cl- influx and unidirectional Rb+ efflux. Both the RVD and hypotonically activated Rb+ efflux were inhibited by the K(+)-channel blocker barium. On the other hand, hypotonically activated Rb+ influx was increased by barium treatment. In sum, the HRPE exhibits volume-sensitive transport mechanisms over a range of volumes from 190 to 390 mosmol/kgH2O. Cultured HRPE possess hypertonically activated Na-K-Cl cotransport, hypotonically activated K-Cl cotransport, and a barium-inhibitable hypotonically activated K+ efflux pathway.