Three healthy adult males were restricted to complete bed rest for periods of 30–36 weeks. Urinary calcium excretion was elevated throughout bed rest, averaging 61 mg./day above the base-line value of 193 mg./day. Maximum urine calcium excretion occurred during the seventh week and was 136 mg./day above the base-line value. Fecal calcium excretion was also increased during bed rest. Sweat calcium was unchanged and represented only 2 per cent of calcium output. Mean calcium balances for the three subjects during bed rest were −202, −207, and −254 mg./day. The measured calcium loss during the entire bed rest period averaged 4.2 per cent of the estimated total body calcium. Calcium balance became more normal but remained negative during the three-week period of reambulation. Phosphorus excreted in the urine and phosphorus balance patterns were similar to calcium patterns. Serum calcium and phosphorus levels did not change appreciably during bed rest, but both levels fell during reambulation. Urinary hydroxyproline and pyrophosphate were mildly elevated during bed rest and fell with reambulation. Gamma ray transmission scanning of the os calcis revealed large losses of mineral during bed rest. The decreased mass in the central portion of this bone ranged from 25 per cent to 45 per cent. Mineral reaccumulated in the central os calcis following reambulation at a rate similar to its rate of loss during bed rest. Bone dissolution during bed rest may occur to a greater extent in weight-bearing bones than in the remainder of the skeleton, and the process appears to be reversible.