NH3 (46.2-27,700 nmol/kg/day; 7 days) decreases heart rate modestly starting at 46.2 nmol/kg/day, but the effect was lost at >2920 nmol/kg/day. NH3 has no effect on the cholesterol-lowering action of 46.2 nmol/kg/day T3, but it inhibits the tachycardic and TSH suppressant effects up to the 924 nmol/kg/day dose.