Progesterone and oestrogen administered to rats ovariectomized during gestation will successfully maintain pregnancy. However, the altered steroid environment created by ovariectomy may result in enlarged placentae (Carpent, 1962; Csapo & Wiest, 1969; Hooverman, Meyer & Wolf, 1970) without influencing the decidua basalis (Stewart & Leathem, 1968). Normal placental growth involves cell division and the synthesis of protein and nucleic acids until day 17. At this time DNA synthesis stops but RNA continues to accumulate (Winick & Noble, 1966). When giant placentae were induced in steroid-treated ovariectomized rats, protein and nucleic acid content exceeded the normal (Leathem, Stewart & Sweki, 1970). This finding prompted an examination of the quantity and distribution of ribosomes. Sprague—Dawley rats weighing 200 g were mated and day 1 of pregnancy determined by the presence of sperm in the vaginal smear. Oestrone 0·5 μg) and progesterone (4·0 mg) were injected s.c. daily from days 3–20 in intact