AbstractThe brain of the young rat contains appreciable amounts of desmosterol (24‐dehydrocholesterol). The peak desmosterol concentration is seen during the first week of life and only traces of this sterol are found at 21 days. The spinal cord also contains some desmosterol. Rat brain desmosterol is distributed in the white matter, gray matter and cerebellum and occurs in the same proportion to cholesterol in each of these brain fractions. Rat brain contains a small amount of sterol ester but no appreciable amounts of desmosterol are present in this fraction.Studies carried out in intact animals injected either intraperitoneally or intracerebrally with mevalonic acid‐2‐14C or glucose‐U‐14C indicate the biosynthetic origin or brain desmosterol. Rat brain slices (1舑20 day old) incubated in suitably fortified medium convert sodium acetate‐2‐14C and glucose‐U‐14C to desmosterol, whereas brain slices from adult rats yielded no radioactive desmosterol under similar conditions. When labeled desmosterol was incubated with young rat brain slices, it was converted to cholesterol.When pregnant rats were treated with triparanol (20 mg/kg/day) during the course of their pregnancy, they either resorbed the fetuses or gave birth to small, stillborn litters. The brains of the progeny of triparanol treated mothers contained large amounts of desmosterol as well as another sterol which may be ख7,24‐cholestadiene‐3ॆ‐ol.