To test the hypothesis that normal age-related limitations in cardiovascular homeostasis may become clinically significant under stress, the cardiovascular response to postural change was assessed in six young and six old healthy subjects before and after modest diuretic-induced sodium depletion. Before diuresis, systolic blood pressure was maintained (from 110 ± 4 to 113 ± 6 mm Hg) while heart rate increased 22% (from 67 ± 2 to 82 ± 5 beats/min) at 3 minutes after 60-degree upright tilt in young subjects. After a significant diuretic-induced weight reduction and natriuresis, the young again maintained systolic blood pressure (from 110 ± 4 to 110 ± 6 mm Hg) and increased heart rate 49% (from 68 ± 2 to 101 ± 5 beats/min;p< 0.05, compared with prediuresis values) in response to the same postural stimulus. During the prediuresis tilt, the older subjects showed no change in systolic blood pressure (from 132 ± 4 to 134 ± 6 mm Hg) and a 9% increase in heart rate (from 68 ± 3 to 74 ± 2 beats/min). After a similar significant weight reduction and sodium loss, the older subjects showed a significant reduction in systolic blood pressure (from 132 ± 6 to 108 ± 6 mm Hg;p< 0.05) and a 17% increase in heart rate (from 69 ± 4 to 81 ± 3 beats/min;p< 0.05) during tilt compared with values in young subjects. Three of six elderly subjects noted postural symptoms. These results suggest that, although the healthy old may appear well compensated under optimal conditions, decreased cardiovascular reserve renders them susceptible to postural change following mild sodium depletion.