The maximum impact stress at the contact of the vocal folds achieved during the oscillation cycle was estimated in phonation using an aeroelastic model of voice production. Relations of impact stress to the lung pressure, fundamental frequency of self-oscillations, prephonatory glottal width, sound pressure level generated at the end of the glottis and vibration amplitude of the vocal folds were studied. Using the fundamental frequency, prephonatory glottal width, lung pressure and airflow rate values found in normal speech, maximum impact stress values of 2-3 kPa were obtained. The results fall well within the limits reported for excised canine larynges and human subjects. Impact stress increased with lung pressure almost linearly after the phonation threshold but reached a plateau when the limit of maximum glottal opening was reached. When the fundamental frequency and lung pressure were kept constant, impact stress appears to fit closely with a parabolic function of prephonatory glottal half-width.