The electrophysiological effects of intravenous administration of diphenylhydantoin (DPH) (5mg/Kg, maximum 250mg) were studied in 20 patients with sinus node dysfunction (SND). DPH shortened spontaneous cycle length (SCL) in 3 patients and lengthened it in 3. Maximum corrected sinus node recovery time (max CSRT) was prolonged in 7 patients and shortened in 3. Estimated sinoatrial conduction time was prolonged in 3 patients and shortened in 2 of 10 patients in whom this measurement was possible. However, these changes were not statistically significant. Neither were there any significant changes in PA, AH and HV intervals nor refractory periods of the atrium, the AV node and the ventricle. DPH prolonged SCL and/or max CSRT in 9 of 20 patients with SND, and it was suggested that DPH has depressant effects on the sinus node in some patients with SND. Thus, this drug should be used with caution in the treatment of ventricular arrhythmias accompanied by sinus node dysfunction.