The presence of brain potentials related to the end of a sustained isometric contraction prompted this study. Potentials related to the beginning of simple ballistic flexion of the index finger were compared with isometric contraction, potentials related to the termination of the isometric contraction, and potentials related to ballistic extension of the reposed index finger. The electroencephalograms recorded in 10 healthy volunteers from scalp positions C3+2, Cz, C3-2 were averaged time-locked to the appropriate mechanograms. Both negativity preceding and positivity following execution of movement were better expressed with simple flexion movements. The negativities preceding all four types of movements were of comparable magnitude whilst the positivities differed significantly. These findings were considered as supporting the ideas of general facilitation preceding preparation for action and specific feedback information following execution, the changes being more prominent over postcentral leads. The existence of potentials related to the termination of voluntary sustained isometric contraction implied a separate command for antagonistic muscles.