Two common polymorphisms in the beta1-adrenergic receptor gene, Ser49Gly and Arg389Gly, are associated with variable antihypertensive response to metoprolol. We sought to determine whether similar pharmacogenetic associations were present with the negative chronotropic response phenotype to metoprolol. Metoprolol was titrated in 54 untreated hypertensive patients to achieve blood pressure control. We found no association between either resting or exercise heart rate at baseline (untreated) or in response to metoprolol by codon 389 genotype. In contrast, when compared by codon 49 genotype, Ser49 homozygotes had significantly higher resting heart rates at baseline (untreated) than Gly49 carriers (82+/-10 versus 74+/-11 bpm, respectively, P=0.016). When corrected for plasma concentration, we found no difference in reduction in exercise heart rate in response to metoprolol between Ser49 homozygotes and Gly49 carriers (0.75+/-0.11 versus 0.57+/-0.17%/ng/ml, respectively, P=0.37). However, if one fails to account for plasma concentration, trends toward a significant difference in heart rate reduction are seen between Ser49 homozygotes and Gly49 carriers (31% reduction versus 25% reduction, P=0.05). Our data suggest that neither the beta1-adrenergic receptor Arg389Gly, nor the Ser49Gly polymorphisms are associated with variable negative chronotropic response to metoprolol. In addition, our data highlight the importance of measuring metoprolol concentration in order to account for variable pharmacokinetics and avoid misinterpretation of the data.