1. The distribution of ammonia between extra- and intra-cellular compartments of the brain was evaluated in anaesthetized rats exposed to sustained hyperammonaemia and superimposed acute hypo- or hyper-capnia.
2. It is concluded that the concentration gradient for ammonia between arterial plasma and cerebrospinal fluid (CSF) cannot be explained by pH-dependent non-ionic diffusion of ammonia. A continuous uptake by the brain tissue of ammonia from the CSF is postulated and possible mechanisms are discussed.