Colchicine, injected ip in doses of 0.05–0.2 mg/100 g caused significant hypocalcemia in intact and nephrectomized rats. Colchicine prevented the rise in serum calcium following the injection of parathyroid hormone to thyroparathyroidectomized animals. Studies with 45Ca indicated that colchicine produced hypocalcemia by suppressing bone resorption. (Endocrinology90: 1589, 1972)