The aim of this experiment was to study the effects of atherosclerosis on the cardiovascular hemodynamics in conscious condition and in response to hemorrhagic hypotension. Arterial pressure was monitored, via a catheter chronically inserted into the aortic arch, for six hours in seven normal and eight WHHL (Watanabe-Heritable Hyperlipidemic) conscious rabbits, sampled at one sec intervals using AD-converter and stored in a digital computer. Percentage frequency distribution curve, mean and standard deviation of the mean arterial pressure for six hours (21, 600 data points) were calculated for each string of pressure data. Mean and standard deviation were pooled for the seven normal rabbits and the eight WHHL rabbits, respectively. Mean and standard deviation of mean values were 117±10 mmHg in the WHHL rabbits, and 87±7 mmHg in the normal rabbits. They showed significant difference (p<0.05) . Mean and standard deviation of standard deviations were 9.5±1.8 mmHg in the WHHL rabbits and 6.5±1.4 mmHg in the normal rabbits. Difference in mean standard deviation between the normal and WHHL rabbits was significant (p<0.05) . Thus, the WHHL rabbits were slightly but significantly hypertensive and their systemic pressure was less controlled. This suggests that the pressure control capacity of the rapidly acting arterial pressure control system, i.e., the baroreflex system, deteriorates. In order to quantify the capacity of the rapidly acting arterial pressure control system, the open-loop gain (G) of the system was estimated in fifteen WHHL rabbits, aged 3 to 31 months. Twenty-five normal Japanese white rabbits, aged 6 to 30 months, were used as a control. After being anesthetized by intravenous injection of Nembutal, the rabbits were bled by 2 ml /kg body weight within 1 to 2 sec through a catheter inserted into the aortic arch. The arterial pressure change after this quick hemorrhage was monitored via a catheter placed in the aortic arch for more than 2 min. G was calculated as (ΔAPI/ΔAPs) -1, where dAPI was the immediate fall and ΔAPs the steady-state fall. The pulse pressure, ΔAPI and ΔAPs increased and G decreased with aging in the WHHL rabbit. These results indicate that the progress of atherosclerosis with aging in the WHHL rabbit decreases the vascular compliance and impairs the arterial pressure restoring function of the rapidly acting arterial pressure control system.