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Suckling rats were killed at various intervals after intraperitoneal injection of acetate-1-14C and their brain sterols were analyzed by column, thin-layer, paper, and gas-liquid chromatography. The crude sterol (to which carrier zymosterol was added) was separated by column chromatography into cholesterol, desmosterol, and zymosterol fractions, and the specific activities of the recovered digitonides were determined.The zymosterol fraction, mainly carrier, was not uniformly labeled, in that the trailing half of the peak had a higher specific activity than the leading half. Evidence obtained suggests that this carbon activity was present in one or more sterols resembling zymosterol (Δ8,24-cholestadienol), Δ7,24-cholestadienol, and Δ7,5.24-cholestatrienol. The desmosterol and cholesterol were also carbon-labeled. The time course of the distribution of carbon activity among the above fractions indicated that the zymosterol fraction is a precursor of the desmosterol and that the desmosterol is, in turn, a precursor of the cholesterol.The data suggest that, in the developing brain of the rat, the course of the transformation of cholesterol precursors into cholesterol is influenced by the presence of at least two slow steps, one involving the conversion of Δ7- and Δ8-compounds to Δ5-compounds and the other, the reduction of the Δ24-unsaturation.