Experimental work in the dog and cineradiography of swallowing in man (at 25 frames/sec) has been performed in order to study the changes that occur in the cricopharyngeal sphincter during swallowing. It has been shown that the sphincter in the normal closed state provides a zone of elevated pressure which prevents the reflux of oesophageal contents into the pharynx. This resting condition is seen to alter in the dog during a reflexly induced swallow, when the sphincter first relaxes and then contracts. These actions are dependent on at least one of the nerves to the cricopharyngeus muscles being intact and in continuity between the pharynx above and the sphincter below. The bolus on reaching the relaxed sphincter is able to pass through under the action of gravity and thrust from the tongue above. Cine films do not show that movement of the cricoid forwards opens the sphincter ahead of the bolus.