A progressive change in level of the hepatic microsomal mixed function oxidase (MFO) system and the inducibility of cytochrome P-450 (P-450) were measured during pregnancy in Sprague-Dawley (SD) rats. The content of P-450 decreased gradually from 5th day of conception whilst that of NADPH P-450 reductase increased at the same time. The levels of these two enzymes, however, returned to those of non-pregnant rats after delivery. Pregnancy did not have any significant effect on P-450 dependent aryl hydrocarbon hydroxylase (AHH) and ethoxycoumarin deethylase (ECDE) including even the levels of cytochrome $b_5$ ($b_5$) and NADH cytochrome $b_5$ reductase ($b_5$ reductase). Pregnancy did not trigger the induction of any specific P-450 isozyme. The inducibility of P-450 by 3-methylcholanthrene (MC) was similar to that of non-pregnant rats, and the induction of P-450 c isozyme has been caused only by MC. These results suggest that although P-450 content was slightly decreased, overall xenobiotic metabolizing ability of microsomal MFO and the inducibility of P-450 by administration of drug such as a MC, were not significantly changed during pregnancy in SD rats.