Crab urinary bladder appears to possess several morphological and functional simmilarities to vertebrate renal proximal tubule. Sections of intermolt rock crab bladder accumulated p-amino hippuric acid (PAH) by a process that was concentrative (60 minute tissue-to-medium ratio (T/M) for 10 micromolar PAH averaged 24), Na-dependent, powered by glycolytic metabolism, and inhibitable by other organic anions. Initial bladder section uptakes exhibited saturation kinetics and a double-reciprocal plot of uptake vs.concentration yielded a single line with a Km of 70 micromolar and a Vmax of 5 nanomol per miligram tissue per hour. Chlorophenol red and bromocresol green (BCG) competitively inhibited PAH uptake. When bladder sheets were mounted in a flux chamber, they exhibited a large, net lumen-to-serosa (L -> S) flux in 10 micromolar PAH that was abolished by 1 mM BCG. The small unidirectional S -> L flux was not BCG-inhibitable. Bladder sheets exhibited PAH T/M >1 after luminal, but not serosal, exposure. BCG only reduced bladder sheet T/M after luminal exposure. The data are consistent with uphill, Na-dependent, and carrier-mediated entry of PAH at the luminal membrane and nonmediated exit at the serosal membrane.