We measured the venous and arterial pressure, as well as the rate of secretion and content of cyclic AMP and high energy phosphate compounds, of the rectal gland of the anaesthetized dogfish, Squalus acanthias (L.). Intravenous infusion of isotonic solutions produced a very large increase in the rate of secretion by the rectal gland. The increase in secretion was preceded by an increase in venous blood pressure, but arterial blood pressure was not modified. Injections of small doses of veratridine stimulated gland secretion when given in the vicinity of the heart but not when given in the dorsal aorta. During volume expansion the creatine phosphate and ATP content of the gland were markedly reduced, while ADP and AMP as well as cyclic AMP content were increased. We conclude that: volume expansion leads to the release of a message that activates adenyl cyclase in the gland; the increased venous pressure may be the initial signal in the sequence that leads to the release of the activating messenger; there is a receptor mechanism in the atrial and cardiac region that triggers the sequence that activates glandular secretion; the reduction in the content of high energy phosphate compounds during volume expansion is caused by an increase in energy expenditure, probably due to gland secretion.