The mechanism of ammonium uptake was studied in Pseudomonas aeruginosa, measuring the uptake (transport and metabolism) of [14C]methylammonium (MA). This ammonium analogue was not utilized for growth, but unmetabolized MA was accumulated to intracellular concentrations about 30 times higher than those in the medium. Most of the MA taken up, however, was rapidly metabolized to gamma-N-methylglutamine, which could be removed from the cells by the addition of ammonium. Uptake of MA exhibited distinct optima at pH 7.0 and 35 to 40 degrees C and depended on metabolic energy, as indicated by the inhibitory effect of various metabolic poisons. Growth with ammonium as nitrogen source resulted in the repression of MA uptake, whereas high uptake rates were observed with nitrate or after incubation without nitrogen source. These results suggested that the ammonium/MA uptake system is subject to nitrogen control in P. aeruginosa.