The synthesis of [20,21-13C2]-pregnenolone (7) from androst-5-en-3β-ol-17-one (1) is described. Labelled carbons were introduced by two procedures, namely, condensation of 1 with K13CN and Grignard reaction of nitrile derivative 5 with [13C]-methylmagnesium iodide. Location of labels was confirmed by 13C-NMR spectroscopy.