The proteins of the smooth and rough endoplasmic reticulum from fetal, immature, and adult male rats were compared after incorporation of two radioactively labeled precursors, 14C-labeled amino acids and δ-aminolevulinic acid-3H by means of gel electrophoresis. The labeling patterns indicated that protein components present in two major electrophoretic bands underwent significant synthesis in fetal tissue while three actively incorporating protein bands were noted in adult tissue. Although the uptake of the amino acids-14C decreased for the smooth and rough elements of the endoplasmic reticulum as a whole during liver development, the qualitative patterns were not significantly different in adult and fetal livers. The over-all incorporation (disintegrations per minute per milligram protein) of the heme precursor into the smooth and rough elements also did not change with development. However, a change was noted in the distributional electrophoretic patterns with development. The estimation of molecular weight (by disc electrophoresis) and the incorporation of the heme precursor suggested the similarity of the two major protein bands to cytochrome P-450 and cytochrome b5, components of the endoplasmic reticulum, thought to be involved in the mixed-function oxidase system. The evidence indicated that in fetal liver, at a time when the oxidase capability was low, the amino acid incorporation into these two protein groups was the same as in the adult. The incorporation of the heme moiety, however, was different, decreasing in the cytochrome b5 region and increasing in the cytochrome P-450 region during development. These results correlate with the increase in oxidase activity associated with liver development.

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