The effects of Isuprel administered by means of a mistometer were studied in 8 patients with heart disease. Recordings were made at varied heart rates using atrial pacing. In addition, the functional and effective refractory periods were measured with the use of the extra stimulus technique. Four inhalations of the mistometer were administered to each patient (0.5 mg). The AH interval was 100 +/- 13 msec before and 93 +/- 23 msec 5 minutes after the Isuprel administration (p less than 0.05). Isuprel significantly reduced the AH interval with atrial pacing. The AH interval at an atrial pacing rate of 100/minute was 148 +/- 70 msec before and 131 +/- 51 msec after Isuprel (p less than 0.05). Isuprel had no significant effect on the HV interval. The effective and functional refractory period of the atrioventricular node was not significantly changed after Isuprel. The heart rate and blood pressure showed no significant change after Isuprel. No cardiac arrhythmias were seen. (The systemic absorption of the drug was probably minimal.) Isuprel administered by means of the mistometer can improve conduction through the atrioventricular node. Nevertheless, the potent bronchodilating effects of Isuprel and the absence of tachycardia and cardiac arrhythmias does attest to the safety of this agent.