The metabolism of various dietary sterols and the effects of an azasteroid on sitosterol metabolism in the free-living nematode Caenorhabditis elegans was investigated. The major unesterified sterols of C. elegans in media supplemented with sitosterol, cholesterol or desmosterol included 7-dehydrocholesterol (66.5%, 40.5%, 31.2%, respectively), cholesterol (6.7%, 52.3%, 26.9%), lathosterol (4.4%, 3.6%, 1.7%) and 4a-methylcholest-8(14)-en-3B-ol (4.2%, 2.1%, 3.8%). Esterified sterols, representing less than 20% of the total sterols, were somewhat similar except for a significantly higher relative content of 4a-methylcholest-8(14)-en-3B-ol (23.3%, 23.4%, 10.6%). Thus C. elegans not only removes the substituent at C24 of dietary sitosterol but possesses the unusual ability to produce significant quantities of 4a-methylsterols. When C. elegans was propagated in medium supplemented with sitosterol plus 5 ug/ml of 25-azacoprostane hydrochloride, the azasteroid stronly interfered with reproduction and motility of C. elegans and strongly inhibited the delta24-sterol reductase enzyme system; excluding sitosterol, the major free sterols of azacoprostane-treated C. elegans were cholesta-5,7,24-trien-3B-ol (47.9%), desmosterol (9.4%), fucosterol (2.1%) and cholesta-7,24-
dien-3B-ol (2.0%). These 4 sterols are likely intermediates in the metabolism of sitosterol in C. elegans.