Nine normal subjects were asked to make either a brisk isometric pinch of a force transducer held between the forefinger and thumb, or to hold a pinch for approximately 15 s and then release the force suddenly without any overt antagonist contraction. EEG activity was averaged about the onset or offset of EMG activity, and movements were made in the subjects' own time. All subjects found the task simple. The EEG activity preceding offset of contraction (offset cortical potential) was significantly smaller in lateral leads than that seen before onset of contraction. Midline activation was similar in both tasks. We suggest that in the onset task, motor cortex activity related to the act to be performed contributes substantially to the EEG potentials in lateral leads. This activity is absent in the offset task which requires only withdrawal of tonic input to motor cortex. Midline activity, common to both onset and offset tasks, could reflect timing, attentional, or other processes. The results are discussed in relation to previous data from an isotonic relaxation task.