Regional cerebral blood flow was measured in 21 normotensive subjects during supine rest and during head-up tilt to 70 degrees. The results showed significant and consistent regional cerebral blood flow changes in the frontal areas with lower relative flow distribution values (percentage of mean flow) during head-up tilt than during supine rest. The lower frontal flow distribution values during tilt were not related to habituation, to repeated measurements, or to the estimated level of arterial CO2 which was derived from expired end-tidal CO2 levels. None of the subjects had orthostatic hypotension and there was no significant difference in mean hemispheric blood flow between lying down and standing up. There was no significant gender difference in regional cerebral blood flow, although female subjects tended to have higher mean hemispheric flow than males in both postures. It remains to be established whether the flow decreases in the frontal cortex are caused by cerebral functional factors or by haemodynamic mechanisms.