1. After a fixed weight-related dose given intravenously, plasma diazepam concentrations were significantly lower in 11 cirrhotic patients than in controls matched for age and sex, in the 4 h after diazepam administration but not thereafter. 2. When measured at a single fixed time point, a greater proportion of the drug was in the unbound from in the plasma of cirrhotic patients, but non-bound diazepam concentrations were not significantly different in the two groups. 3. Several psychomotor tests showed that cirrhotic patients, although having significantly impaired liver function, did not as a group have increased sensitivity to diazepam compared with their matched controls. 4. Only those cirrhotic patients who at the time of drug administration had impaired cerebral function, as judged by baseline performance of psychomotor tests, showed increased sensitivity to the effects of intravenous diazepam. 5. Psychomotor tests, particularly the Reitan trail test, seem more useful than tests of liver function or drug metabolism for identifying those patients with liver cirrhosis at risk of excessive sedation after diazepam administration.