Atrioventricular nodal (A-H) and infranodal (H-Q) conduction times were determined in 22 patients undergoing intracardiac electrophysiologic studies. These measurements were determined independently both on line from a digital oscilloscope and from paper recordings at a rate of 100 mm/sec. There was no significant difference and excellent correlation between mean A-H (91.39 +/- 29 versus 95.44 +/- 32 msec) and H-Q (53.48 +/- 14 versus 50.52 +/- 15 msec) intervals determined by these two methods. The digital oscilloscope enables virtually instantaneous highly accurate (+/- 1 msec) timing of intracardiac events and rapid and more efficient processing of data. Potential applications during electrophysiologic studies include precise monitoring of drug effects on atrioventricular conduction and refractoriness, on line analyses of complex rhythm disturbances, and ready storage of data for computer retrieval and analyses.