The incorporation of C14-valine into the mitochondrial proteins of loach oocytes in vivo is seven times greater during the period of slow growth and the early phase of the period of greatest growth than the incorporation of this amino acid into mitochondrial proteins at the end of the period of greatest growth. Incorporation of the tagged amino acid into the mitochondrial proteins is inhibited by chloramphenicol. Cycloheximide inhibits tagged-precursor incorporation into the proteins of the nucleus and 14, 000-g supernatant and into the mitochondrial proteins.