Ionic regulation by the gills of the freshwater-adapted Chinese crab, Eriocheir sinensis, was examined. The balance of uptake and loss of NaCl in crabs living in freshwater was established. Urine production was measured directly by cannulating the nephropores. Daily urinary loss of Na+ is equivalent to 16% of the haemolymph Na+ content and is substantially higher than that based on data from indirect measurements reported in the literature. Weight and area of anterior and posterior gills are proportional to body weight. The role of the gills in compensating urinary loss by uptake was determined by analysing changes in Na+ and Cl- concentrations in the external medium in which isolated perfused gills were suspended. In posterior gills, salt loss is quantitatively balanced by NaCl net uptake from an external concentration of 1.3 mmol l(-1) NaCl upwards. The transport constant (Kt) for half maximum saturation of net uptake and saturation of NaCl uptake are 1.5 mmol l(-1) and 4 mmol l(-1), respectively. In contrast to previous studies in which tracer fluxes or transepithelial short-circuit currents were determined, our method of direct ion determination shows that no net uptake of Na+ or Cl- occurs in posterior gills in the absence of the respective counter ion, or when uptake of one ion is blocked by a specific inhibitor. Net uptake of Na+ and Cl- was about equal. We conclude that the uptake of the two ions is coupled. The properties of the branchial ion uptake of E. sinensis correlates with the distribution of this crab in river systems.