Cerebral impedance plethysmography (REG) is a non-traumatic, reproducible method of continuously monitoring aspects y of the cerebral circulation. Its use in more than 200 healthy subjects aged 6–70 years was investigated, measurements being applied to three characteristics of the standardized averaged REG wave form. It was found that inflow angle and percent rise time changed progressively and significantly as chronological age advanced but amplitude was not so affected. Menstrual status, sex and hemispheric sidedness were not significant variables. The role of REG as a monitor of the ageing process in man is discussed.