Motor potentials (MEPs) evoked by focal (figure-of-eight coil) transcranial magnetic stimulation of the left motor cortex were recorded from the right opponens pollicis (OP) and flexor digitorum superficialis (FDS) of 14 normal subjects during different motor tasks. Changes in motor cortical excitability under behavioural conditions presumably connected with premotor and supplementary motor area (SMA) activation were investigated by comparing the size of the MEPs obtained during: (1) rest, (2) mental calculus, (3) repetitive left thumb-to-index opposition, (4) mental simulation of the same task with the right hand, (5) sequences of left thumb-to-fingers opposition, and (6) mental simulation of the same sequences with the right hand. MEP size significantly increased in both muscles during sequential movements of the left hand and sequence simulation with the right hand, but not during mental calculus or actual or simulated repetitive movements. The H-reflex evoked in the OP and FDS muscles by electrical stimulation of the median nerve (at wrist and elbow, respectively) under the same experimental conditions did not show significant modifications. The increase in MEP size during non-routine actual or imagined sequences of finger movements supports the view that the SMA is activated under these conditions and that it exerts a direct facilitatory influence on the motor cortex.