The cardiac electrophysiological effects of metoprolol were studied in ten patients (pts) aged 22-51 years undergoing intracardiac stimulation studies for paroxysmal palpitations. The following measurements were made: (1) basic sinus cycle length (SCL); (2) sinus node recovery time (SRT) following overdrive pacing; (3) atrio-His (A-H) and His-ventricular (H-V) conduction intervals during regular atrial pacing; (4) effective refractory periods of the atria (AERP), A-V node (AVERP) and ventricular myocardium (VERP); and (5) A-V nodal functional refractory period (AVFRP). All measurements were repeated 10-20 min after a slow bolus intravenous injection of metoprolol (0.2 mg/kg body weight). Results were analyzed by the Wilcoxon Signed Rank test. Highly significant increases in: SRT (932 +/- 166 to 1107 +/- 225 ms, p less than 0.001) and A-H (72 +/- 14 to 103 +/- 14 ms, p less than 0.001) were observed. There were also small increases of lesser significance in SCL (794 +/- 160 59 898 +/- 190 ms, p less than 0.01), AVERP (308 +/- 46 to 341 +/- 66 ms, p less than 0.02), and AVFRP (376 +/- 43 to 416 +/- 59 ms, p less than 0.01). Metoprolol did not affect the H-V interval, the AERP, or the VERP. These results are similar to those of other beta blockers.