An artificial erection was induced in nine patients consulting for erectile dysfunction. Changes in the intracavernous pressure (ICP) and in the integrated EMG of the ischiocavernosus muscle were recorded during voluntary muscular contractions. During such contractions elevations in ICP, varying between 100 and 525 mm. Hg, were recorded. Changes in ICP were always in phase with changes in the integrated electromyogram (EMG) of the ischiocavernosus muscle, and correlations between the duration of changes showed an almost perfect linear relationship between both physiological events. Correlations between maximum changes in ICP and integrated EMG generally showed a positive relationship between both measures. Results are interpreted to suggest involvement of the ischiocavernosus muscle in the process of penile rigidity.