Articular cartilage is avascular and relatively hypoxic, but is habitually studied at ambient O₂ levels. Reduction in O₂ tension, however, inhibits ATP production in articular chondrocytes (Lee & Urban, 1997), and O₂ tension per se profoundly modulates membrane transport in other tissues (Gibson et al.,, 2000). Both may alter ion homeostasis, and hence cell function. pH and Na⁺ are particularly important. Intracellular pH (pH_i) affects matrix turnover, whilst the Na⁺ gradient, maintained by the Na⁺/K⁺ pump, is used for many secondary active processes including the major pH regulatory process, Na⁺/H⁺ exchange (NHE). Here we investigate the effects of O₂ tension on chondrocyte Na⁺ and H⁺ homeostasis. Cartilage slices were taken from bovine and equine fetlock joints of animals humanely killed for other purposes (under Home Office guidelines). Chondrocytes, isolated by collagenase digestion, were incubated at 21% or 0% O₂ for 10 min before assaying ion transport at 37^oC. pH_i was determined fluorimetrically with BCECF (Wilkins & Hall, 1992), using NH₄Cl to alter pH_i. Ouabain-sensitive ⁸⁶Rb influx was used as a measure of Na+/K+ pump activity. Salines were buffered with HEPES (10 mM), in nominal absence of CO₂ / HCO₃^–. Steady state pH_i was 6.93±0.15 and 6.89±0.07 in normoxia and anoxia, respectively; buffering capacity, assessed by NH₃ rebound, was 38.61±4.10 mM.(pH unit)^-1 in O₂ vs 28.79±8.97 in N₂; amiloride (100μM)-sensitive H⁺ efflux during recovery from acid load was 1.47±0.16 mol.(l cells.h)^-1 in O₂ and 1.12±0.25 in N₂ (all means±S.E.M., n>4). NHE activities were also confirmed using ²²Na⁺ influx (data not shown). None of the above changes was statistically significant (tested using Student’s t-test). Na⁺/K⁺ pump activity, however, was 31.8±5.7 and 59.5±9.9 nmol.(10⁶ cells.h)^-1 in oxygenated and deoxygenated cells, respectively (n=3; p<0.05). ATP production by chondrocytes is largely glycolytic. It is reduced in anoxia, a “negative” Pasteur effect, correlating with inhibition of metabolic events such as matrix synthesis, although steady state ATP levels are largely maintained (Lee & Urban, 1997). We show that H⁺ regulation and NHE activity were little affected by short term anoxia but Na⁺/K⁺ pump activity was substantially elevated. Thus reduction in ATP consumption is not uniform across all cell functions. We speculate that, unlike matrix synthesis, H⁺ and Na⁺ homeostasis are critical for cell survival, accounting for their relative protection during anoxia.