The present study compared the relative expression pattern of 10 orthologous CYP forms from families 1-4 in cDNA panels of human and mouse fetal and adult tissues. Except for CYP1A2, all of these CYPs exhibited specific patterns of expression during mouse ontogeny, suggesting possible involvement in development. Cyp1a1 and Cyp2r1 were the only two of the orthologs to be expressed only in the E7 mouse; Cyp2s1 was expressed in all stages, including E7, while Cyp2e1 appeared only at E17. Highest expression of the individual CYPs in the different late term human fetal tissues was: thymus, CYP1B1 and CYP2U1; liver, CYP2E1; brain, CYP2R1, CYP1A1 and CYP4X1; and lung, CYP4B1 and CYP2W1. In general, the level of individual human CYP transcripts was lower in the fetal than the corresponding adult tissues. The pattern of expression in adult mouse and human tissues was fairly similar for CYP1A1, CYP1A2, CYP1B1, CYP2S1, and CYP2U1 orthologs.