Changes in sensitivity of the rapidly acting arterial pressure control (AP) system with aging were investigated. Two catheters, one for pressure measurement and the other for inducing hemorrhage from the aorta, were chronically implanted in 25 rabbits from the same colony (aged 6 to 30 months). A few days after the operations, each animal was quickly bled (2 ml/kg body weight) while it was conscious. The hemorrhage experiment was repeated 16 times and the mean arterial pressure responses were sampled with an A/D-converter and stored in a digital computer, the 16 strings of data being pooled for each animal. The overall open-loop gain (G) of the AP-system was estimated from the individually pooled responses. In the present study, aging exerted no significant effect on the value of G (7.1) as evaluated by Kruskal-Wallis' non-parametric one criterion variance analysis (p greater than 0.05). The reflex sensitivity of the AP-system over the pressure ranges used in this experiment thus appears to be unaffected by aging over the range from 6 to 30 months.