SUMMARY An organ culture system was used to examine the effects of oestrogens on the response of 5-day-old mouse calvaria to parathyroid hormone (PTH). Parathyroid hormone released calcium and phosphate from the bone and this was associated with an increase in glucose consumption, an accumulation of citric acid and an inhibition of citrate oxidation. Oestradiol, oestriol, oestrone and ethinyl oestradiol all inhibited the PTH-induced release of calcium. The accumulation of citrate was prevented without the PTH-induced block on citrate oxidation being removed, and this was explained in terms of a reduction in glycolysis. Oestradiol, oestriol and oestrone appeared to be of equal potency. However, ethinyl oestradiol was active at much lower doses but appeared to be toxic at higher levels.