The effects of aging on cardiac output in the intact, unanesthetized rat were determined in 50 l2-mo.-old and 45 24-mo.-old male Wistar rats. Heart rate, phasic, and mean blood pressure were measured through an implanted intra-arterial catheter. Cardiac output determinations were made by S0RbCI indicator dilution technique. in 30 young and 26 old rats studied at rest the heart rate was slightly higher (p < .05) and the mean blood pressure was slightly lower (n < .05) in the old than in the young rats. The cardiac index and the stroke index were significantly lower in the old than in the young rats (p < .005 and n < .001, respectively). in 20 young and 19 old rats studied after beta-adrenergic blockade (propranolol I mg/kg), there were no age differences in heart rate or blood pressure. However, after beta-adrenergic blockade, cardiac index and stroke index remained significantly lower in the old than in the young rats (n < .025 for both indices). Thus, the decline in cardiac output and stroke volume with age was not related to differences in sympathetic nervous system tone.