The effects of different concentrations of the hypocholesterolemic drug AY9944, an inhibitor of ΄7-reductase, on organotypic cultures of fetal mouse spinal cord, were studied by light and electron microscopy. Exposure to 10-8M produced no observable changes. After 6 hours exposure to 10-4M, dense membrane-bound inclusions were occasionally observed in neurons. After 24 hours exposure to 10-4M, numerous cytoplasmic inclusions occurred in neurons, glia and macrophages. The form of these inclusions varied but were predominantly of two types; concentric, loosely-packed lamellae resembling membranous cytoplasmic bodies (MCB) of Tay-Sachs disease and irregular dense bodies. They were identical to those observed in our previous in vivo study. Prolonged exposure to the drug at 10-4M caused an increased number of inclusions in all cell types. Eventually the cultures degenerated. The number of inclusions increased for at least 38 days following a 2–5 day exposure to AY9944 at 10-4M. However, by 70 days, although inclusions persisted, the cultures were mostly astrocytic. In long-term cultures, in addition to these inclusions, curved or straight electron-dense paired profiles were seen in some cells, presumably macrophages. Biochemical analysis of cultures exposed to 10-4M revealed the continuous presence of ΄7, 24-cholesta-diene-3β-ol and 7-dehydrocholesterol even after the drug was removed from the cultures.