1. The relationships between the haemodynamic, renal and endocrine changes induced by rapid ventricular pacing were studied in ten chloralose-anaesthetized dogs paced from the right ventricular apex for 60 min at 250 beats/min.
2. Pacing increased mean right atrial and mean pulmonary wedge pressure (P < 0.05), and decreased cardiac output and mean arterial pressure (P < 0.05).
3. Coronary sinus atrial natriuretic peptide (ANP) concentrations were approximately fourfold greater than arterial concentrations; both increased markedly during pacing (P < 0.01). Plasma concentrations of arginine vasopressin and plasma renin activity did not change signficantly.
4. Urine flow and free water clearance increased during the latter 30 min of pacing (P < 0.05). There was no significant change in sodium clearance despite high sustained concentrations of ANP.
5. Without the availability of specific inhibitors of ANP release or action, we are unable to exclude the possibility that ANP may have prevented sodium clearance from otherwise decreasing during rapid ventricular pacing. Nevertheless, the dissociation between elevated ANP concentrations and natriuresis in this study indicates that a rise in ANP concentrations per se is not sufficient to produce a natriuresis.