The incubation of Δ7-cholestenol with a 10000g supernatant or 105000g microsomes in the presence of tritiated water is studied. The reisolated Δ7-cholestenol contained up to 0·67g.atom of tritium/mole. This result can best be explained by assuming the reversibility of the reaction Δ8-cholestenol ⇌ Δ7-cholestenol.