The purpose of this study was to assess the potential of transcranial Doppler (TCD) ultrasonography for detecting selective changes in cerebral blood flow velocity during mental activity. Mean flow velocity was continuously and simultaneously measured in the right and left middle cerebral arteries in 26 healthy right-handed young subjects at rest and during performance of verbal and visual-imaging mental tasks. These two mental tasks produced significantly different effects on the right and left sides: the verbal task produced a higher increase of flow velocity (mean absolute difference above baseline +/- SD) with respect to the basal values in the left than in the right middle cerebral artery (5.56 +/- 3.8 cm/s vs 1.25 +/- 3.1 cm/s); the visual-imaging task was accompanied by a higher increase in the right than in the left middle cerebral artery (3.92 +/- 3.3 cm/s vs 1.52 +/- 3.1 cm/s)--analysis of variance (ANOVA) three-fold interaction side of recording x task x condition, F = 25.67, p < .0001). Heart rate, blood pressure, and skin conductance showed comparable increases during performance of both mental tasks. Respiratory activity showed no modification during the mental activity with respect to the rest phase. These results demonstrate the possibility of delivering specific functional information via bilateral TCD and suggest wider utilization of this noninvasive technique in neuropsychological studies.