Saturation diving involves the exposure of humans to elevated partial pressure of oxygen (PO2) and high ambient pressure. The present study is part of a research program that focuses on how such conditions affect basal cellular functions. C3H/10T1/2 Cl 8 mouse embryo fibroblasts were exposed to 20-80 kPa O2 in a He-O2 mixture at 0.1 and 5.0 MPa ambient pressure for 24-72 h. Elevated PO2 had severe toxic effects on the cells, and there was an additional effect of high pressure on net cell growth. A persistent reduction of cell growth rate after the end of exposure to He-O2 was noted, suggesting genetic effects. We observed no effects of the ambient pressure per se in this respect. High PO2 increased the cellular glutathione level reaching a plateau approximately 100% above control at a PO2 of 60 kPa. No alteration of the glutathione redox status was observed, and high ambient pressure per se had no significant effect on the cellular glutathione content. The increased glutathione content did not completely protect the cells against toxic injury of high oxygen levels.