1. We evaluated a method of measuring cardiac baroreflex sensitivity (BRS) derived from spontaneous changes in systolic pressure (SP). SP was measured from the ECG signal in seven conscious, resting dogs. 2. Beat-to-beat changes in PI (dPI) were positively correlated with beat-to-beat changes in SP (dSP) in all dogs tested, suggesting spontaneous baroreflex function. The slope of the regression of dPI on dSP was used as an index of spontaneous BRS. 3. The spontaneous BRS was abolished by hexamethonium, atropine and bilateral carotid sinus denervation. Low dose atropine sulphate produced a paradoxical increase in spontaneous BRS, which has been observed in other studies. The spontaneous BRS was positively correlated with the average pulse interval in resting dogs. 4. Random modulation of heart rate after vagotomy failed to reproduce the strong positive correlation between dSP and dPI; this demonstrated that the correlation was not the result of mechanical coupling between heart rate and arterial blood pressure. 5. The BRS was measured pharmacologically in six dogs using a bolus injection of a vasoconstrictor. The pharmacological BRS was positively correlated with the spontaneous BRS measured after the bolus injection. 6. Finally, the spontaneous BRS was negatively correlated with the average arterial pressure in resting dogs. We conclude that the spontaneous BRS is a useful quantitative indicator of baroreflex function in conscious resting dogs.