Brain edema, a lethal complication of acute liver failure (ALF), is believed to be largely cytotoxic due to the swelling of astrocytes. Ammonia, a principal neurotoxin in ALF, has been strongly implicated in the development of the brain edema. It was previously shown that treatment of cultured astrocytes with ammonia (5 mM NH₄Cl) results in cell swelling. While ammonia continues to exert a direct effect on astrocytes, it is possible that ammonia can affect other neural cells, particularly microglia. Microglia are capable of evoking an inflammatory response, a process known to contribute to the brain edema. We therefore examined the potential role of microglia in the mechanism of ammonia-induced astrocyte swelling. Conditioned media (CM) derived from ammonia-treated cultured microglia when added to cultured astrocytes resulted in significant cell swelling. Such swelling was synergistically increased when astrocytes were additionally treated with 5 mM ammonia. CM from ammonia-treated microglia also showed significant release of oxy-radicals and nitric oxide into the CM. CM from ammonia-treated microglia containing Tempol (a superoxide scavenger) or uric acid (a peroxynitrite scavenger) when added to astrocytes resulted in marked reduction in the cell swelling. Together, these studies indicate that microglia contribute to the ammonia-induced astrocyte swelling by a mechanism involving oxidative/nitrosative stress.