The effect of ammonia on water space of astrocytes in culture was determined as a means of studying the neurotoxicity of ammonia in fulminant hepatic failure (FHF). Treatment of primary astrocyte cultures obtained from neonatal rat cortices with 10 mM NH4Cl for 4 days resulted in a 29% increase in astrocytic water space, as measured by an isotopic method utilizing 3-O-methyl-[3H]-glucose. This effect was time- and dose-dependent. The ammonia-induced swelling was reversible as the water space in cultures treated with 10 mH NH4Cl for 3 days, and then returned to normal culture media for 1 day, was similar to control cultures. These findings suggest that elevated levels of ammonia lead to astrocyte swelling and may contribute to the brain edema in FHF.