The induction doses of thiopentone in eight elderly male patients (60--80 years) were significantly lower (p less than 0.01) than in eight young male patients (ranges 3.09--5.06 mg/kg body weight in the elderly and 4.88--8.11 mg/kg in the young). The sleep concentrations in venous as well as in arterial blood did not differ between the two groups. In the elderly the arterial concentrations were significantly higher than the venous (p less than 0.05). A three compartment open model was used to describe the disappearance of the drug from the venous blood. Significant increases were found in V3 (means 145.9 and 51.2 litres) and in T 1/2 beta (means 781 and 427 minutes) in the elderly. The redistribution rate constant, k1.2, was decreased in the elderly (p less than 0.05). By impedance cardiography 5 minutes after sleep mean percentages of the pre-induction value of cardiac output were 94.6% in the young and 87.0% in the elderly (p less than 0.05). In the young a correlation was found between cardiac output and induction dose (4 = 0.85, p less than 0.01).