The electrophysiological effects of prenalterol, a new beta-receptor agonist, in a dose of 100 micrograms/kg body weight given intravenously for 5 min, were studied in 13 patients with signs of sinus node dysfunction and/or conduction defects within or distal to the atrioventricular (AV) node. In nine patients with signs of sinus node dysfunction a significant reduction was found in corrected sinus node recovery time, on an average by 1955 +/- 640 ms (-61%, P less than 0.05) and in atrial refractoriness, by 61 +/- 21 ms (-20%, P less than 0.05). Similar but insignificant changes were also seen in the four patients with normal sinus node function. In eight patients with AV nodal dysfunction, a significant increase was found in the Wenckebach point, by 51 +/- 10 b.p.m. (+52%. P less than 0.01) and a decrease in the AH interval, by 23 +/- 9 ms (-14%, P less than 0.05). AV nodal refractoriness tended to decrease by 115 +/- 58 ms (-24%, NS). Similar changes were found in the five patients with normal AV conduction. Heart rate increased in all 13 patients, on an average by 28 +/- 5 b.p.m. (+44%, P less than 0.001) and systolic blood pressure by 18 +/- 8 mmHg (+13% P less than 0.01). In conclusion, prenalterol increased sinus node automaticity and atrial and AV nodal conductivity, but did not improve infranodal conduction. Thus, the drug might be useful in the treatment of patients with sinus node dysfunction as well as in patients with spontaneous or induced atrioventricular conduction abnormalities.