Developmental changes in bufuralol 1'-hydroxylation activity, which is known as a typical activity of cytochrome P-450 (CYP)2D isoforms, in the liver of rats were investigated. The catalytic activities of hepatic microsomes increased with development especially from 3 to 7 weeks. Eadie-Hofstee plots of bufuralol 1'-hydroxylation were obtained for monophasic kinetics (Km: 0.037 microM) at 1 week and for biphasic kinetics (Km: 0.051 and 6.4 microM) at 7 weeks of age. Quinine completely inhibited bufuralol 1'-hydroxylation activity of hepatic microsomes of 1- and 7-week-old rats. These results indicated that at least two kinds of CYP2D isoforms, which differ markedly in their affinity for bufuralol, were present at 7 weeks of age and that the CYP2D isoform that had low affinity for bufuralol was expressed with development. To assess the affinity of CYP2D isoforms for bufuralol, the kinetic properties of CYP2D1, 2D2, 2D3, and 2D4 expressed in yeast cells were investigated. The Km value of CYP2D2, 0.044 microM, was extremely small compared with that of the other rat CYP2D isoforms. We further investigated developmental changes of CYP2D isoform mRNA by reverse transcription-polymerase chain reaction. CYP2D3 mRNA increased with development although CYP2D1 and 2D2 mRNA were not changed. The CYP2D4 mRNA was not detected. These findings indicated that CYP2D2, which had high affinity for bufuralol, was expressed in immature and mature rats, but CYP2D3, which had low affinity for bufuralol, was expressed only in mature rats.