Addition of mercuric chloride at concentrations which resulted in an overall binding level of about 8 mmoles Hg/l packed cells and above caused a breakdown in the permeability of the cell membrane as indicated by a net efflux of internal K(+). Below this level in region of 2 mmoles Hg/l packed cells the rate of K(+) transfer across the cell surface was stimulated without affecting the internal K(+) level. Maintainence of the stimulation was dependent both on time and dose. Enhancement of the rate of K(+) turnover was associated with a fast component of the inorganic mercury uptake which could be removed by washing with cysteine. The mercury stimulated K(+)/K(+) exchange was inhibited by low temperature, by the uncoupler CCCP and the energy transfer inhibitor DCCD. Overall binding concentrations of inorganic mercury below 0.5 mmoles/l packed cells had no effect on the K(+) transport system. In contrast to mercuric chloride, methyl mercuric chloride over similar concentration ranges did not seem to induce a breakdown in the permeability barrier or directly interact with the K(+)/K(+) exchange but more likely influenced the latter by inhibiting intracellular processes.