The investigation of active Na-K transport inhibition by mercury is difficult to perform in a cell because of the presence of numerous other membrane and intracellular proteins modifiable by mercury. Thus purified Na-K-adenosinetriphosphatase (ATPase) molecules performing active transport in an artificial membrane are required to demonstrate unequivocally the inhibition of active transport by mercury. We made use of a single population of Na-K-ATPase liposomes filled with ATP and Na to show mercury inhibition of active 86Rb transport mediated by both the inside-out and right-side-out pumps in the same liposome. The effect of HgCl2 on the Na-K-ATPase in cell-like and reversed orientation was measured in comparison with convallatoxin. A dilution series showed that 10 microM externally added HgCl2 inhibited the active 86Rb transport at the cytoplasmic side first; at 50 microM both pump populations were blocked, indicating either membrane permeation by HgCl2 and inhibition at the internal intracellular domains or onset of extracellular action at higher HgCl2 concentration. The results show that the metal-binding interface of Na-K-ATPase molecule is profoundly implicated in active ion transport and that the intracellular part of the Na-K-ATPase molecule presents the primary target for mercury action.