Spectral analysis was used to identify correlated sinusoidal frequency components in left and right side diaphragm electromyographic (EMG) recordings from human subjects during voluntary deep inspirations. In 31 of 33 subjects bilaterally correlated high-frequency oscillations were found in broad or narrow bands in the range of 60-84 and 16-40 Hz. To determine if such oscillations were associated also with bilaterally symmetric, phasic, voluntary activation of nonrespiratory muscles, we obtained EMG signals from left and right masseter muscles during clenching of the jaw; left and right sternomastoid muscles during lifting of the head against gravity; and left and right biceps muscles during lifting of a weighted bar. Weakly correlated frequency components, mainly at frequencies below 60 Hz, were found in the left and right masseter EMGs on at least one trial from 12 of 17 subjects. No bilaterally correlated frequency components were found during phasic contraction of biceps and sternomastoid muscles. Power spectra of biceps EMGs, however, sometimes exhibited peaks indicative of oscillations that were not bilaterally correlated. In nine subjects, correlated frequency components in the 60-84 Hz range were found in intercostal EMGs from the axillary region of the fifth interspace during voluntary deep inspirations but not during postural contractions. We conclude that high-frequency oscillations in the range of 60-84 Hz in diaphragm and intercostal EMGs are associated particularly with respiratory activation of respiratory muscles. These results support the hypothesis that high-frequency oscillations may be a manifestation of control of muscular contraction via a central pattern generator.