Treatment of rats with one electroconvulsive shock (ECS) per day for 10 days enhanced the hyperactivity syndrome produced by administration of tranylcypromine (10 mg kg-1) and L-tryptophan (50 mg kg-1) given 24 h after the final shock. Similar enhancement was seen whether the shock was alternating sinusoidal or direct current (fractionated), whether it was given through unilaterally or bilaterally placed electrodes and whether or not a neuromuscular blocking agent (fazadinium) was used. Five shocks spread over 10 days or 8 shocks spread over 17 days were similarly effective, whilst 8 shocks in 1 day were ineffective. Therefore when ECS are given to rats in ways similar to those in which electroconvulsive therapy is given to patients with depression, enhancement of behavioural responses to increased 5-HT function is produced.