High concentrations of phosphate drastically inhibited the aerobic growth ofAzospirillum brasilense in liquid medium having a limiting concentration of combined nitrogen. No growth inhibition occurred if a high concentration of combined nitrogen was present. Aerobic growth occurred only in the presence of a very low amount of phosphate and this could be enhanced by adding norepinephrine. Addition of norepinephrine enhanced the O2 uptake by cell suspensions by almost a factor of 3. All of the intermediates of the tricarboxylic acid metabolic cycle were actively oxidized in cell-free extracts and their rate of oxidation increased in the presence of norepinephrine.