Epithelial sheets (including cuticle) from posterior gills of the freshwater-adapted euryhaline crab Eriocheir sinensis were obtained according to the method of Schwarz and Graszynski ((1989) Comp. Biochem. Physiol. 92A, 601-604; (1989) Verh. Dtsch. Zool. Ges. 82, 211 and (1989) Arch. Int. Physiol. Biochim. 97, C45). With external NaCl-saline, the outward-directed short-circuit current (Isc) could hardly be influenced by external amiloride up to 100 mumol/l but was, on the contrary, strictly dependent on apical Cl- (Onken, Graszynski and Zeiske (1991) J. Comp. Physiol. B 161, 293-301). In absence of external chloride an inward-directed, amiloride-inhibitable Isc was observed which depended on external Na+ (thus, Isc approximately INa) in a two-step, saturating mode. The Isc-block by amiloride obeyed saturation kinetics (half-maximal at less than or equal to 1 mumol/l, suggesting apical Na(+)-channels). Only for Na+ concentrations below 100 mmol/l we found an indication for a competitive interaction between Na+ and amiloride at the channel. Current fluctuation analysis revealed the presence of an amiloride-induced relaxation (Lorentzian) component in the Isc-noise (so-called 'blocker-noise'). The Lorentzian parameter-shifts with increasing amiloride concentration indicate first-order kinetics of the blocker with its apical receptor. Using a 'two-state' blocking model we calculated, for amiloride concentrations between 2 and 5 mumol/l, a mean single-channel current of 0.46 pA and a mean channel density of 250.10(6) cm-2.