Prostacyclin (PGI2) has been shown to reduce the occurrence of experimental ventricular arrhythmias. To assess potential beneficial effects in man, the electrophysiological action of PGI2 was studied in 16 non medicated patients. The protocol used in incremental pacing and programmed stimulation in the right atrium and ventricle. This protocol and measurement of effective refractory periods (ERP) were performed before and during the injection of 2.5, 5 and 10 ng kg-1 min-1 of PGI2. The atrial functional refractory period decreased significantly (P less than 0.05); PGI2 had no influence on the occurrence of inducible non-sustained (NS) atrial tachycardias and was responsible for the occurrence of 2 non-sustained atrial tachycardias in 8 patients with inducible atrial echo beats under basal conditions. Thirteen patients did not have inducible ventricular tachycardia (VT) under basal conditions. Non-sustained VT was induced after PGI2 in 4 of them but in only 1 of them after the administration of propranolol. Three patients had inducible VT under basal conditions (1 non-sustained, 2 sustained VT). PGI2 did not prevent the occurrence of VT (1 non-sustained, 1 sustained VT), except in 1 patient with ischaemic-related VT, who had non-sustained VT after PGI2. In conclusion, PGI2 does not seem to have a cardiac antiarrhythmic effect and may increase the atrial and ventricular repetitive response. This effect could be related to an increase of adrenergic tone.