This proposal represents a continuation of previous and ongoing longitudinal studies to characterize the relative contribution of pharmacogenetics and ontogeny to variability in CYP2D6 and CYP3A4 activity between birth and adolescence. Specifically, the overall goal of the research program is to test the hypothesis that genetic variability, rather than ontogeny, is the primary source of variability in CYP2D6 and CYP3A4 activity during adolescence. This study will extend our current series of longitudinal phenotyping studies and will be a cross-sectional study in children aged 7-15 years, studied on seven occasions, 6 months apart.