Two series of acetylenic derivatives of cholesterol were synthesized from stigmasterol and pregnenolone. These compounds carry an acetylenic function at C-22 and were devised with the aim to inhibit the C-22 hydroxylation of ecdysonebiosynthesis by a suicide-substrate mechanism. Two of these compounds (15a, 15f) inhibit the synthesis of ecdysone in follicular cells under in vitro conditions. The inhibition