1. The effects of acute blood volume change in conscious rabbits on a.c. gain of the carotid baroreceptor reflex with respect to heart rate, blood pressure, cardiac output and systemic vascular resistance were studied.
2. With acute, isohaemic increase in blood volume by 20% and 40% the only consistent trend was a decrease in gain for systemic vascular resistance.
3. With acute reduction in blood volume there was a consistent tendency for gain for heart rate to fall. With 20% reduction in blood volume, gain for cardiac output fell but gain for systemic vascular resistance rose and its phase-lag became shorter, so that gain for blood pressure was unaltered. The enhanced gain for systemic vascular resistance was not sustained with 35% reduction in blood volume, so that gain for blood pressure fell.
4. Thus control of blood pressure by the carotid sinus reflex is remarkably unaffected by acute change in blood volume, and is impaired only when there is depression of gain for cardiac output without a concomitant rise in gain for systemic vascular resistance.