K influx into equine red blood cells (RBCs) was measured using 86Rb as a tracer for K under conditions designed to mimic the changes in respiratory blood parameters that occur in vivo during strenuous exercise. The effects on K influx of physiological changes in pH, cell volume, O2 tension (PO2), CO2 tension (PCO2), and bicarbonate and lactate concentrations were defined. Physiological PO2 exerted a dominant controlling influence on the H(+)-stimulated Cl-dependent K influx, consistent with effects on the K-Cl cotransporter, PO2 required for half-maximal activity was 37 +/- 3 mmHg (4.9 kPa). Although RBCs were swollen at low pH, results showed explicitly that the volume change per se had little effect on K influx. Lactate had no effect on volume- or H(+)-stimulated K influxes, nor did bicarbonate or PCO2 affect the magnitude of K influxes after these stimuli or after treatment with protein kinase/phosphatase inhibitors. These results represent the first detailed report of O2 dependence of H(+)-stimulated K-Cl cotransport in RBCs from any mammalian species. They emphasize the importance of PO2 in control of RBC K-Cl cotransport.