1. Liquid ion-exchanger Cl- -sensitive micro-electrodes were used to make continuous measurements of the intracellular Cl activity, aCli, of quiscent sheep cardiac Purkinje fibres in vitro. 2. aCli was higher than that expected from a passive distribution, (which would have been about 5 mM). It was 3--4 times hiable; EC1 was about 35 mV positive to Em. It was over twice as high in the nominal absence of bicarbonate/CO2 (when the buffer-system was HEPES/O2) but was not always so stable, and ECl was about 20 mV positive to Em. 3. Experiments designed to assess the maximum possible error likely to occur in the measurement of aCli showed that this could not be large and that the estimates of ECl were accurate to within 8 mV. 4. The ability of Cl to move down both concentration and potential gradients was established by demonstrating a loss of aCli in Cl-free solutions and a gain when Em was depolarized positive to ECl in high-K solutions. In both cases, the changes were complete within about 100--160 min. 5. The decline of aCli in Cl-free solutions (glucuronate-substituted) was not significantly affected by changes of [Ca]o from 0 to 12 mM or by the depolarizations of Em of up to 60 mV that sometimes occurred in low or zero [Ca]o. 6. Only 2--3 mM-aClo was sufficient to impede substantially the ready loss of aCli in HEPES-buffered solutions. 7. In high-K solutions (45 mM), Cl appeared to be passively distributed since, at equilibrium, Em and ECl differed by less than 2 mV. 8. In HEPES-buffered Tyrode, ECl of quiescent papillary muscle of the guinea-pig was, on average, 39 mV positive to Em. 9. It is concluded that liquid ion-exchanger Cl- -sensitive micro-electrodes are suitable for studying the Cl regulation of sheep Prukinje fibres, and probably of other cardiac tissues. The measurements of resting aCli are quite accurate when using either HEPES or bicarbonate-buffered Tyrode. The results are discussed in relation to estimates of the apparent membrane Cl permeability under various conditions and the possible existence of an inwardly directed 'Cl pump'.