Elevations of the concentration of ammonia in arterial blood to 20 times that of control values, or more, were induced in nine patients by the intravenous administration of an approximately neutral solution of 2 m glycine. During the course of the infusion, increases were observed in uptake of oxygen, output of carbon dioxide, respiratory exchange ratio, ventilation, and arterial pCO2; small reductions in arterial pH accompanied these changes. The net effects observed during glycine administration were metabolic stimulation and relative respiratory depression. A poor correlation (γ = 0.23) was shown to exist between ventilation and the calculated arterial ammonia tension. In addition, in the two patients so studied, the ventilatory response to the inhalation of 5% carbon dioxide in air was decreased during a period in which the concentration of ammonia in the blood was elevated. It is concluded that elevating the concentration of ammonia in the blood to as high as 500 μg/100 ml does not stimulate respiration and may depress it perhaps by changing the pH within chemosensitive cells toward alkalinity. Submitted on February 28, 1961