The ultrastructural response of the rat mammary gland was studied in normal oestrous rats given 0·0 2·5, 5·0 or 10·0 mg perphenazine/kg body weight daily for 5 days starting 12 hr after ovulation. Growth of the parenchymal tissue was largely limited to alveolar development. Tissue from controls receiving only injections of the 0·03 n-HCl vehicle was similar to that from rats at 7 days' gestation, the earliest stage of pregnancy studied. Protein granules appeared in the Golgi complexes in rats given the 2·5 mg/kg dose and the occurrence increased with increasing dose levels, but these granules did not appear in the lumina until the dose reached 5·0 mg/kg. Large lipid droplets appeared in the cytoplasm in rats given the 5·0 mg/kg dose or more, but were not found in the lumina except at the highest dose.
Despite the presence of abundant protein and lipid accumulations in the cytoplasm and lumina in mammary tissue of rats given the 10·0 mg/kg dose and the abundance of free ribosomes in the parenchymal cells, the rough endoplasmic reticulum was grossly underdeveloped as compared with mammary glands removed from pregnant rats 2 days before parturition. Evidence is presented from these and collateral studies to suggest that progesterone levels, elevated by the perphenazine treatment, are responsible for inhibition of rough endoplasmic reticulum development and, thus, the capacity for both protein and lipid biosynthesis.