1. A method for recording carotid sinus diameter in conscious dogs has been developed, using an ultrasound transit time technique. We have used this method to measure the distensibility of the carotid sinus in resting animals, and we have investigated the effect of injecting the vasoactive drug phenylephrine on the mechanical behaviour of the sinus wall. 2. Baroreflex sensitivity was measured in the dogs by the usual method of relating the slowing of the heart to the transient pressure rise induced by the injections of phenylephrine. 3. We have shown that phenylephrine directly reduces the radius of the carotid sinus during the measurements of baroreflex sensitivity. At a dose of 2-4 micrograms/kg, phenylephrine has an enhancing effect on the baroreflex response, possibly resulting from a modification of baroreceptor output.