In Rana oocytes, measurements with chloride-sensitive microelectrodes show that the mean intracellular chloride activity (34.8 +/- 6.3 mM, n = 79) is three times higher than that expected for the passive distribution of chloride ions across the outer membrane (12.4 mM, mean membrane potential -43 +/- 8.8 mV, n = 79). 2. Reuptake of chloride into oocytes depleted by prolonged exposure to chloride-free saline takes place against the electrochemical gradient. 3. Chloride reuptake does not take place in sodium-free solution or in a sodium-substituted potassium-free solution. It is inhibited by bumetanide (10(-5) M) in the bathing medium. 4. The overall stoichiometry of the transport mechanism deduced from simultaneous measurements of intracellular sodium and chloride using ion-selective electrodes is 1Na+:1K+:2Cl-. 5. Ammonium ions substitute for potassium on the cotransporter. 6. In oocytes smaller than 0.9 mm in diameter, exposure to external ammonium causes an alkaline shift in intracellular pH as the NH3 enters and takes up H+ to form NH4+. We propose that chloride-dependent NH4+ transport contributes to the accumulation of NH4+ and causes the 'postexposure' acidification as the intracellular NH4+ releases H+ to form NH3 which is then lost from the cell. 7. In larger oocytes ammonium exposure produces a rapid reduction in pHi which may be explained in part by cotransport-mediated uptake of NH4+. Evidence is also provided for a second chloride-dependent NH4+ transport mechanism and a chloride-independent process.
