Uptake of 10 micromolar p-aminohippuric acid (PAH) by sections of C. borealisurinary bladder was concentrative, saturable (Km 67 micromolar, Vmax 1.7 nanomol per mg tissue per hour), inhibitable by other organic anions and dependent on medium Na and glycolytic metabolism. Bladders mounted in flux chambers exhibited net secretory transport of PAH, with serosa-to-lumen fluxes (J s->l) being about 4 times lumen-to-serosa fluxes (J l->s). In 60-minute flux chamber studies, tissue-to-medium ratios exceeded unity with serosal, but not luminal, PAH. Initial (10 min) fluxes and tissue accumulations (Ac) were measured in the absence and presence of 1-5 mM BCG (bromocresol green; competitor organic anion). With serosal PAH, serosal BGC (1 mM) reduced serosa-to-cell flux (J s->c), Ac, and J c->l by 60-75%. With luminal PAH, luminal BCG (1 mM) had no effect on J l->c, Ac, or J c->s; increasing luminal BCG concentration to 5 mM reduced J l->c, Ac, and J c->s by 40-50%. The data are consistent with a model featuring an inwardly directed pump on the serosal membrane, cellular accumulation, and a facilitated carrier on the luminal membrane.