CEREBROSPINAL fluid (CSF) amino acid neurotransmitter concentrations in 23 patients with acute encephalitis were compared with those in patients with acute brain infarction, multiple sclerosis and controls. The concentration of glutamate was significantly higher in encephalitis (5.2±6.7 μmol/l) and stroke patients (9.6 ±14.2 μmol/l) than in MS patients (1.6 ± 0.9 μmol/l) and controls (1.7 ± 0.8 μmol/l; p < 0.001). The concentration of glycine was significantly higher in encephalitis (11.0 ± 4.7 μmol/l) than in stroke (7.6±3.2 μmol/l) and MS patients (6.3 ± 2.1 μmol/l) or controls (5.6±1.8 μmol/l; p < 0.002). Taurine levels were significantly lower in encephalitis patients than in the other groups (p = 0.04). The correlation of high glutamate levels with poor outcome was almost significant (Kendall tau 0.63, p = 0.06). Our observations suggest that exicitotoxic neurotransmission may play an important role in the series of events that lead to neuronal damage in encephalitis.