A quantitative electron microscopic analysis of human preimplantation embryos in conjunction with [3H]uridine labeling and light microscopic autoradiography revealed significant differences in the fractional volume of some cell organelles between the blastomeres of eight-cell embryos with fully activated extranucleolar and nucleolar transcription and those showing low extranucleolar and no nucleolar RNA synthesis, a pattern typical of four-cell human embryos. The latter type of blastomeres in eight-cell embryos did not show any significant quantitative cytological difference when compared to blastomeres of four-cell embryos. The phenotypical changes accompanying the overall enhancement of the embryonic transcriptional activity (increase in tubules/vesicles ratio and lysosomes, decrease in Golgi apparatus) were due to repartition of intracellular membranes amongst different types of organelles rather than to a noticeable change in the existing equilibrium between total membrane production and degradation.