Base-catalysed decomposition of the 12-tosylhydrazone (II) of hecogenin acetate under aprotic conditions occurs without rearrangement, to give the Δ 11-olefin (III). Hydroxylic solvents favour the formation of the C-nor-D-homo-Δ 13(17a)-olefin (VI). The tosylate (Ia) of the 12β-alcohol derived from hecogenin acetate undergoes solvolysis to give mixtures containing varying proportions of the C-nor-D-homo-Δ 13(17a)-olefin (VI) and the Δ 7a(18)-isomer (IV), depending upon the reaction conditions. Polar solvents and elevated temperatures favour the endocyclic (Δ 13(17a)-olefin, but the exocyclic (Δ 17a(18)-olefinpredominates in solvents of lo NMR evidence is presented in support of the Δ 13(17a)-structure for the endocyclic olefin. Rearrangements of the 12-tosylhydrazone (XVIb) and 12β-tosyloxy (XVIa) derivatives in the pregnane series gave only the endocyclic Δ 13(17a)- and Δ 17(17a)-olefins.