Intracellular Cl activity (aiCl), and intracellular pH (pHi) were measured in isolated sheep cardiac Purkinje fibres using a liquid ion exchanger Cl-selective micro-electrode and a glass recessed-tip, pH-selective micro-electrode. Removal of external Cl (glucuronate substituted) produced a fall in aiCl from about 20 to about 4 mmol/l: the residual level is probably caused by intracellular interference on the Cl-sensitive electrode. Re-exposure of the fibre to increased levels of external Cl produced, in the steady state, increased levels of aiCl. The dependence of steady-state aiCl upon external Cl activity, aoCl, was roughly hyperbolic with 50% recovery occurring at an aoCl of about 9.5 mmol/l. At all levels of external Cl tested, Cl was accumulated to a level much higher than that predicted for passive electrochemical equilibrium. Exposure of a Cl-depleted fibre to various levels of external Cl produced an exponential rise with time in aiCl. The initial rate-of-rise in aiCl was estimated to be a saturating function of aoCl, with a half-maximal effect occurring at an aoCl of about 33 mmol/l. The rate-of-rise was about 10-fold greater than that predicted from constant-field theory using published values for PCl, the Cl permeability coefficient. Steady-state aiCl was essentially insensitive to changes in external HCO3 concentration, [HCO3]o, if these changes were made at a constant external pH, pHo, i.e. when a reduction in [HCO3]o was accompanied by a simultaneous reduction in the partial pressure of CO2, PCO2. In contrast, if PCO2 was maintained constant, then a change in [HCO3]o (thus producing a change in pHo) resulted in an inverse change in aiCl. This change in aiCl was also accompanied by a change in pHi: when aiCl increased, pHi decreased and vice versa. The anion-exchange inhibitor, DIDS (4,4-diisothiocyanato-stilbene disulphonic acid) abolished the effect on aiCl of changes in [HCO3]o and pHo (at constant PCO2). Furthermore DIDS reduced the influence of pHo upon pHi. Both the fall of aiCl in Cl-free solution and the subsequent reuptake of Cl following re-exposure to Cl-containing solution were slowed by a reduction in [HCO3]o (constant pHo, reduced PCO2). Both reuptake and wash-out of Cl were saturating functions of [HCO3]o with half-maximal effect occurring at an [HCO3]o of 1-1.3 mmol/l. The reuptake of Cl was little affected by removal of external Na (bis,2-hydroxy ethyl, dimethyl ammonium substituted). The reuptake of Cl was unaffected by amiloride (1 mmol/l) but slowed by piretanide (1 mmol/l).(ABSTRACT TRUNCATED AT 400 WORDS)