Heat is continually generated in and transmitted through the vocal folds during phonation. Physiological sources are heat generated by muscle contraction, and heat brought to the folds by blood perfusion. Since the vocal folds are a viscoelastic system, a third source of heat is viscous loss in the vocal fold tissues. It has been demonstrated that vibration of the vocal folds results in a measurable increase in their temperature [Titze and Cooper, J. Acoust. Soc. Am. Suppl. 1 71, S55 (1982)], on a scale consistent with a previous theoretical estimate [Titze, Tenth Symposium, Care of the Professional Voice (1981)]. Both muscle metabolism and the viscoelastic properties of tissue are themselves functions of tissue temperature. Consequently, cooling contributes to the stabilization of both the metabolic and mechanical properties of the fold tissues. It has also been found in studies of excised larynges that the surface and internal moisture of the tissues strongly affects the oscillation of the folds. Experiments on excised larynges are described in which the vocal folds are excited by warmed humidified air. The temperature profile of the tissues is measured by an array of inserted fine wire thermocouples and the vibratory pattern of the folds is described in terms of glotto-graphic measurements. The roles of tissue moisture and heat transfer in vocal fatigue and phonation are addressed. [Work supported by NINCDS.]