The acute cerebral haemodynamic effects of modified ECT were monitored by electrical impedance piethysmography in four patients with schizophrenia and three with depression. A total of 20 recordings were made on these patients; of these, 19 involved the following segments, in sequence: a rest period of 25–115 min, a short period in which Atropine, Brietal, and succinylcholine were given by injection, administration of the ECT, monitoring of post-ECT effects for 10 min, and, finally, a recovery period of up to 120 min was recorded. An identical procedure, save for the omission of ECT in one of the patients, formed the 20th recording. Results were considered significant if they were given by a majority of the 19 ECT recordings. No impedance changes took place during the pre-drug resting period nor following the pre-medication drugs. The acute effects of ECT were shown by electrical impedance changes during the 10 min immediately following it. These implied a surging increase in CBF and persisted during the recovery phase. No such changes occurred when ECT was omitted, and the results suggested that these cerebral circulatory changes formed part of the therapeutic action of ECT.