The contribution of scalp blood flow to pulses obtained by cerebral electrical impedance plethysmography (rheoencephalography) has been investigated using a four-electrode (quadripolar) montage with brass disc electrodes 1 cm, 2 cm and 4 cm in diameter. Three states of scalp perfusion were produced by the application of a tourniquet around the occipto-frontal circumference of the head, and blood flow in the scalp was monitored by means of reflection photoplethysmography and an electrical impedance technique. When scalp perfusion was occluded, cerebral impedance pulse amplitude was reduced to 80% of that obtained during normal scalp flow; in the presence of maximal scalp flow as a result of the reactive hyperaemia induced by release of the tourniquet, cerebral impedance pulse amplitude was 30% greater than during normal scalp flow. Altering electrode diameter did not affect the contribution of scalp flow to cerebral impedance pulse size. The effect of changes in scalp flow did, however, increase when a bipolar electrode montage was used instead of a quadripolar montage.