1. The effects of a low calcium diet and of oophorectomy, separately and together, on cortical and trabecular bone mass, have been examined in mature female rats.

2. Calcium deprivation caused a significant decrease of weight, cortical cross-sectional area and ratio of cortical to total area in the femur, it significantly reduced the volume of trabecular bone and increased the percentage of osteoid surface in the tail vertebrae, and in addition increased the urinary excretion of phosphate and, initially, of hydroxyproline.

3. Oophorectomy caused similar though smaller changes in trabecular bone and urine, whereas the effects of oophorectomy on cortical bone were greater on a low calcium intake than on a normal intake.

4. The ash weight of the femora, expressed as a percentage of the total dry weight, was unaffected by calcium deprivation or oophorectomy alone but was significantly reduced when the two occurred together.

5. The percentage of resorption surfaces in the vertebrae tended to increase on the low calcium diet and after oophorectomy on the normal diet but decreased after oophorectomy on a low calcium diet.

6. It is concluded that oophorectomy and calcium deficiency each reduce bone mass in the adult rat but the greatest effect is seen when they are combined.

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