Some ventricular arrhythmias can be related to a decrease in the level of potassium (K) and/or magnesium (Mg). Because adrenergic stimulation decreases serum K+ and Mg++, we decided to investigate the effects of a beta-receptor agonist, isoproterenol, on serum K+ and Mg++, and their consequences on the induction of tachycardia. Programmed atrial and ventricular stimulation was performed in 95 patients before and during infusion of 1.6 micrograms.ml-1 of isoproterenol. During isoproterenol infusion, 61 patients had no inducible tachycardias (group I) and 34 had inducible sustained tachycardias (group II): 16 of them (group IIA) had inducible sustained supraventricular tachyarrhythmias and 18 (group IIB) had inducible sustained ventricular tachycardia. Serum K+ and Mg++ were measured at the end of stimulation in the control state and during isoproterenol infusion. The basal values in groups I and II did not differ (3.8 +/- 0.38 vs 3.86 +/- 0.39 mEq.l-1 for K+, and 20.18 +/- 2.68 vs 19.83 +/- 1.63 mg.l-1 for Mg++). Isoproterenol infusion induced a significant (P < 0.001) hypokalaemia in all groups and a decrease in serum Mg in group II: there was a significant decrease in serum Mg++ (P < 0.05) in group IIA (19.55 +/- 1.7 vs 20.4 +/- 4.6). The decrease in serum Mg++ in group IIB (18.9 +/- 1.55 vs 19.32 +/- 1.63) was not significant. However the serum Mg++ level during isoproterenol infusion was significantly lower in group IIB than in group I. In conclusion, the infusion of isoproterenol was responsible for a significant hypokalaemia, which did not explain the induction of tachycardia.(ABSTRACT TRUNCATED AT 250 WORDS)