Dose response curves have been constructed for the chronotropic effects of small, rapid intravenous injections of isoproterenol sulfate. In 28 studies performed on 16 normal adults, significant intersubject differences in "receptor sensitivity" were found. In one subject studied on four occasions the dose responses were highly consistent. No adverse effects were encountered. The significance of these findings is discussed in relation to claims of abnormal "receptor sensitivity" in patients with asthma and the hyperkinetic heart syndrome.