Two novel epimeric N-oxides, (-)-thalimonine N-oxide A and(-)-thalimonine N-oxide B isolated as mixture from Thalictrum simplex were separated by reverse-phase HPLC. The structures of the epimers were established by spectral analysis and confirmed by synthesis. The IR spectral absorption band of Me-N+-O- group of each epimer was ascertained. The known aporphine alkaloid (+)-leucoxylonine was isolated for the first time from a member of the Ranunculaceae, together with the new (+)-leucoxylonine N-oxide.