1. The effect of subcutaneous administration of bovine growth hormone on calcium metabolism of nearly mature wether sheep has been studied by a combination of a radioactive technique and a nutrient balance technique. 2. Administration of growth hormone resulted in a significant increase in the rates of absorption of Ca, accretion of Ca into bone, resorption of Ca from bone and skeletal retention of Ca, and also in the sizes of the exchangeable Ca pools. 3. Retention of phosphorus was also significantly increased. 4. These changes suggest that the effect of growth hormone was to alter the Ca metabolism of nearly mature wethers to resemble that of younger, more actively growing animals. 5. Results are consistent with the theory that oestrogens may alter Ca metabolism of wethers by increasing growth hormone production.