Heart rate, stroke volume, cardiac output, its distribution and organ blood flows were measured, using the microsphere technique (15 micron in diameter microspheres), in 5, 12 and 24 months old conscious male rats. Each rat was then infused with isoproterenol (0.2 microgram/kg.min) and the measurements were repeated. It was found that resting cardiovascular values do not change within the age range studied, with the exception of the renal blood flow. After infusion of isoproterenol, the heart rate and cardiac output increased in the youngest group while only smaller increase in heart rate and no significant changes in cardiac output were found in the oldest group. Similarly, organ blood flow increased after isoproterenol in all organs studied in the 5 months group, with significance levels reached in the case of blood supply to the right and left ventricles, total heart, skin and carcass. In contrast, infusion of the same dose of isoproterenol in the 24 months old animals did not elicit any significant changes in the regional blood flow. Thus, the response to beta-adrenergic stimulation is depressed with advancing age. This finding of decreased in vivo response to beta-adrenergic stimulation may be a mechanism for the altered cardiovascular performance during stress seen in aged animals and man.