SUMMARY1. Double-barrelled Cl--sensitive microelectrodes have been used to measure the intracellular Cl-activity (a',) and membrane potential (Em) in rat lumbrical muscles. The mean Cl-equilibrium potential (Ecl), calculated from the measured aC1 in sixty fibres, was 29 + 25 mV (S.D. of an observation) less negative than Em. The value of a', was higher than would be expected for a passive distribution, by a mean 1P4 + 1 2 mm. The mean Em was -59-5 + 8-2 mV. 3. If lack of selectivity of the Cl--sensitive ion exchanger and the intracellular presence of interfering anions, assumed to be responsible for the apparent al1 recorded in Cl--depleted fibres, were also responsible for the apparently non-passive Cl-distribution recorded under normal conditions, the difference between the calculated EC1 and Em would increase at more negative potentials. This was not observed over a range of Em values between -46 and -84 mV. 4. Inhibition of the Cl-permeability by application of 9-anthracene carboxylic acid (9-AC) resulted in an immediate rise in ai1 and hyperpolarization. An ai1 up to 40 mm higher, or eleven times higher, than that predicted by a passive distribution was recorded. Application of 9-AC after depletion of intracellular Cl-in Cl--free solution had no effect on either the apparent ai1 or Em.5. It is concluded that Cl-ions are actively accumulated by the skeletal muscle fibre and that the Cl-distribution therefore normally exerts a depolarizing influence.6. In the presence of 9-AC and nominal absence of CO2 and HC03-, readdition of Cl-to Cl--depleted fibres resulted in a substantial rise in a', and a small, maintained 7. Neither application of CO2 and HCQ3-nor application of DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid) had any effect on the accumulation of Cl-ions. This suggests that Cl--HC03-exchange is not involved.8. Removal of Na+ or K+ resulted in a limited rise in a'1 such that EC1 approximated to Em. Removal of both cations had no greater effect than the absence of only one. Readdition of the cations caused an immediate accumulation of Cl-ions and divergence of Ec1 from Em. This was largely inhibited by the presence of frusemide. These results suggest that a Na+, K+, Cl-co-transport mechanism is responsible for the inward transport of Cl-.