26 patients who had undergone right heart catheterization were enrolled in the study. The average age was 51.7 ± 15 years. Half the patients showed coronary artery disease at selective coronary angiography. His bundle recordings and atrial pacing were performed before and 10 min after 1 or 1.5 mg/kg i.v. tiapamil. In addition, arterial blood pressure was recorded. P-R interval increased from a mean value of 153 ± 36 to 168 ± 49 ms (p < 0.05) due to an increase in the A-H time from 88 ± 19 to 97 ± 23 ms (p < 0.05). Arterial blood pressure and heart rate decreased significantly. These changes were more pronounced in patients with coronary artery disease. In the groups with the higher dosage, the differences from the control values were greater than in the patient groups receiving 1.0 mg/kg. Sinus node recovery time tended to increase in all groups but the differences did not reach significance. Patients with the ‘sick sinus syndrome’ were not studied.