SUMMARYPlasma and urine calcium and phosphate were determined in nine patients before and after oophorectomy and in eleven patients before and after simple hysterectomy. Oophorectomy was followed by a significant rise in the mean plasma and fasting urine calcium, and a mean rise of 37 mg in the 24 hr calcium which, however, was not significant. Hysterectomy alone caused no significant change in plasma or urine calcium. In both groups only small changes were observed in plasma and urine phosphate and these did not reach the level of significance. The mean urine hydroxyproline/creatinine ratio rose significantly in six patients after oophorectomy but did not change after hysterectomy. It is suggested the rise in plasma and urine calcium following oophorectomy is due to increased bone resorption which does not occur after simple hysterectomy.