1. A study has been made of the muscular basis of the respiratory pumps in the dogfish by recording electromyograms, pressures and movements simultaneously.

2. Electrical activity in the muscles takes place more or less synchronously during the phase when the oro-branchial and parabranchial cavities are decreasing in volume. Besides being active at this time the adductor mandibulae has a tonic phase of activity which accompanies expansion of the branchial region.

3. There is a slight delay between the activity of the muscles in the branchial region such that those more anteriorly placed precede the posterior ones. All muscles of a given constrictor sheet are active synchronously regardless of their dorsal, lateral or ventral position.

4. During normal resting ventilation no electrical activity was recorded in the hypobranchial musculature. Activity was recorded in these muscles, however, during hyperventilation and also when the fish was made to bite by stimulating the inside of its mouth. In the latter activity the pharyngeal cavity is expanded by contraction of the coraco-muscles and subsequently the adductor mandibulae rapidly closes the jaws.

5. In a fully anaesthetized dogfish the relaxed position of the skeleton is with the mouth open and branchial region expanded. It is concluded that ventilation is primarily achieved by contraction of the superficial constrictor muscles, which produces an increase in pressure in the oro-branchial and parabranchial cavities. Water is forced through the gills because the pressure in the oro-branchial cavity exceeds that in the parabranchial cavities during both this phase and the expansion phase which succeeds it. Expansion is produced when the constrictor muscles relax by the elastic recoil of the visceral skeleton. This action is aided by tonic activity in the adductor mandiibulae.

6. During swimming of certain sharks the whole branchial region remains in a relaxed condition and water enters the mouth. The volume entering could be regulated by variations in the amount of tonic activity in the adductor mandibulae.

7. It is suggested that the importance of the lateral plate musculature during normal ventilation is because it is the phylogenetically older system. Only later in phylogeny did the myotomic component of the head musculature (hypobranchial muscles) become concerned with respiration.

On leave from the Zoological Laboratory of the University, Groningen, The Netherlands. Supported by a grant of the Netherlands Organization for the Advancement of Pure Research (Z.W.O.).