It has been pointed out by Ramsey that metallic ammonium (NH4) may beimportant in determining the properties of Uranus and Neptune. Thepressure-density relation for this metal at pressures of importance in thisconnection is calculated by quanta} methods. The transition pressure forthe phase transition from mixed crystals of ammonia and hydrogen (NH3 and-½ H2 ) to ammonium metal is also investigated and found to be almost certainlyless than 250 000 atmospheres, so that the metallic phase will persist throughoutthe bulk of the two planets mentioned.