The effect of ascorbic acid on cell size and ascorbic acid transport was studied in hyperoxic astrocytes. Subcultured rat astrocytes plated on poly-L-lysine-coated coverslips or on plastic dishes were exposed to serum-free culture medium and 20% or 42% ambient oxygen for 48 h. Vehicle (homocysteine) or L-ascorbic acid was added to the medium at 0 and 24 h. Cell size and relative optical density of glial fibrillary acidic protein-positive astrocytes were measured by a computerized imaging system. Cells on the dishes were used for ascorbic acid transport studies. Hyperoxia significantly increased the cell size of astrocytes, and this effect was inhibited by ascorbic acid. The rate of L-[14C]ascorbic acid Na(+)-dependent uptake was also inhibited by hyperoxia in vehicle-treated cultures but not in ascorbic acid-supplemented cultures. These results indicate that the presence of ascorbic acid during the hyperoxic episode can prevent astrocytic cell swelling and preserve membrane transport function.