[en] 25-³H- and 26-¹⁴C-labelled (24S)-24-ethylcholest-5-en-3 β-ol (clionasterol) were synthesized from (24S)-24-ethylcholesta-5,25-dien-3β-ol. These labelled substrates were mixed and administered, together with the hypocholesterolemic agent, triparanol citrate, to Tenebrio molitor larvae. The ³H label from the clionasterol substrate was retained in both the desmosterol and the cholesterol isolated from the larvae. Location of this ³H label in the desmosterol showed that dealkylation of the clionasterol involved ³H migration from C-25 to C-24. A possible mechanism for dealkylation is presented. (author)