In the mitochondria of animal steroidogenic tissues, cytochrome P450SCCencoded by the CYP11A1 gene catalyzes the conversion of cholesterol into pregnenolone-the general precursor of all steroid hormones. In this work we study the steroid metabolism in transgenic tobacco plants carrying the CYP11A1 cDNA encoding cytochrome P450SCCfrom the bovine adrenal cortex. The transgenic plants under investigation markedly surpass the control wild-type plants by size and are characterized by a shortened period of vegetative growth (by rapid flowering); their leaves contain pregnenolone-the product of a reaction catalyzed by cytochrome P450SCC. The level of progesterone in transgenic tobacco leaves is higher than in the control plants of the wild type. The seeds of the transgenic plants contain less (24R)-brassinosteroids than the wild-type tobacco plants. The results obtained indicate that the synthesis of an active P450SCCcytochrome in transgenic Nicotiana tabacum plants has a profound effect on steroid metabolism and is responsible for the specific phenotypic features of transgenic plants bearing CYP11A1 cDNA. © 2010 Pleiades Publishing, Ltd.