1. The effect of parathyroidectomy on the composition of rat milk was studied in animals maintained on diets varying in calcium and protein content. 2. The main effect of parathyroidectomy on milk composition was an increase in the total milk solids. However, maintenance of the plasma calcium at the control level by removal of litters and provision of dietary calcium abolished this effect. 3. The effect of parathyroidectomy on the calcium content of the milk solids depended on the magnitude of the decrease in plasma calcium and on the availability of dietary calcium and protein.