Mutant alleles CYP2D63 and CYP2D64 were detected by PCR in 33 individuals. All examinees were given 50 mg methoprolol (orally). Plasma concentrations of the drug and its major metabolite was determined by high performance liquid chromatography and arterial pressure and heart rate were monitored. The rate of methoprolol metabolism was significantly lowered in subjects with CYP2D64 mutation. This determined blood pressure decrease and bradycardia in these subjects after drug intake.