In recent years, understanding of normal jaw-muscle function has been enhanced by detailed descriptions of their complex internal architecture and of the functional activity of single motor units (SMUs). The lateral pterygoid muscle, however, has been poorly studied, although it is thought to play an important part in the control of jaw and jaw-joint movement. The present study is the first of a series of SMU studies to clarify the normal function of this muscle. The aims were to demonstrate (a) the unequivocal isolation of SMU activity from one or two verified recording sites within the lateral pterygoid, and (b) that these SMUs can be recorded reliably together with condylar movement during simple command jaw movements. Recordings of SMU activity were made with fine-wire electrodes from sites within the superior and inferior heads of the right lateral pterygoid during biting or command lateral jaw movements and combined with recordings of condylar and mid-incisor point movement. Recording sites were verified by computed tomography. In four young adults, the activities of 17 SMUs were reliably discriminated at seven recording sites within the lateral pterygoid. The units could be recorded during repeated trials of the same movement throughout a recording session with no appreciable change in amplitude or waveform. Units could also be discriminated simultaneously at separate recording sites--one in the superior head and the other in the inferior head. These data demonstrate that SMU activity can be recorded from verified sites within the lateral pterygoid simultaneously with condylar movement during command jaw movements.