The underlying mechanisms of neuronal dysfunction in head injury are multifactorial and not fully understood. Recent experimental evidence indicates that excitatory amino acids--for example, glutamate--may play a key part in secondary neuronal damage after head injury. A patient with severe head trauma was monitored for extracellular amino acids by intracerebral in vivo microdialysis. Very high concentrations of glutamate, glycine, and gamma-aminobutyric acid were recorded in the microdialysate specimens collected over three hours. In vivo microdialysis in head injury may prove to be a useful tool in understanding the pathogenesis of cell death in head trauma.