Kidneys from rats without a pituitary gland have lower transamidinase activity. Giving growth hormone to these rats increases the enzyme activity. Other hormones don't affect it. The enzyme assay can measure growth hormone activity. Loss of enzyme activity after pituitary removal is due to food deprivation and high creatine levels, which return to normal in 7 days. Only growth hormone increases transamidinase activity after this time. Hormones may directly affect transamidinase synthesis or activation.