The conversion of pregna-4,9(11)-diene-17α,21-diol-3,20-dione 21-acetate ( I) and 17,21-diacetate ( VI) by Nocardioides simplex VKM Ac-2033D was studied. The major metabolites formed from I were identified as pregna-1,4,9(11)-triene-17α,21-diol-3,20-dione 21-acetate ( II) and pregna-1,4,9(11)-triene-17α,21-diol-3,20-dione ( IV). Pregna-4,9(11)-diene-17α,21-diol-3,20-dione ( III) and pregna-1,4,9(11)-triene-17α,20β,21-triol-3-one ( V) were formed in minorities. Biotransformation products formed from VI were pregna-1,4,9(11)-triene-17α,21-diol-3,20-dione 17,21-diacetate ( VII), pregna-1,4,9(11)-triene-17α,21-diol-3,20-dione 21-acetate ( II), pregna-1,4,9(11)-triene-17α,21-diol-3,20-dione ( IV), pregna-1,4,9(11)-triene-17α,21-diol-3,20-dione 17-acetate ( VIII), pregna-1,4,9(11)-triene-17α,20β,21-triol-3-one ( V). The conversion pathways were proposed including 1(2)-dehydrogenation, deacetylation, 20β-reduction and non-enzymatic migration of acyl group from position 17 to 21. The conditions providing predominant accumulation of pregna-1,4,9(11)-triene-17α,21-diol-3,20-dione 21-acetate ( II) from I and pregna-1,4,9(11)-triene-17α,21-diol-3,20-dione 17-acetate ( VIII) from VI in a short-term biotransformation were determined.