A small-oscillation theory is developed for determining threshold conditions for onset and release of vocal fold vibration as a function of lung pressure, tissue viscosity, glottal aperture, and surface wave velocity in the mucosa (vocal fold cover). It is found that oscillation threshold lung pressure increases in proportion to mucosal wave velocity. In other words, phonation initiates at lower pressures (less effort) if the vocal fold cover is lax and propagates slow surface waves. Experimental evidence on excised dog larynges confirms that a stiffer cover (at greater vocal fold lengths) requires greater lung pressure. The theoretical prediction that increased glottal aperture (spreading of the vocal folds, as in breathy voice) also increases the oscillation threshold lung pressure has been more difficult to verify experimentally, but increased tissue viscosity produces an observable increase. Applications to phonetics and voice care are cited. [Work supported by NINCDS.]