Organ selectivity of beta sympathetic blockade with propranolol was studied in 6 normal men by comparing the cardiovascular and respiration responses during isoproterenol infusions before and after propranolol. Beta sympathetic blockade was achieved with propranolol and was considered present when there was no heart rate (HR) response to isoproterenol during an infusion tenfold greater than that which raised HR 25% during a control period. During blockade there was no change in HR or systolic or diastolic blood pressure during isoproterenol infusions. There was a consistent (p less than 0.05) rise in resting ventilation (+17%), oxygen consumption (+9%), and carbon dioxide production (+15%) with low-dose (raised HR 10% before blockade) isoproterenol infusion during blockade. These respiratory effects of low-dose isoproterenol during cardiovascular blockade were quantitatively similar to that before blockade. With infusion that raised HR 25%, there was a further increase in VE, VO2, and VCO2 before blockade but no further increase during beta blockade. Changes in acid-base status did not explain the increase in VE during blockade. We conclude that there are differences between effectiveness of propranolol blockade of the cardiovascular system and of the respiratory system.