An experiment was designed to resolve two largely unaddressed questions about the turnover of N in soils. One is the influence of microbial growth rate on mobilization and remineralization of cellular N. The other is to what extent heterotrophic immobilization of NO(3)(-) is controlled by the soil concentration of NH(4)(+). Bacteria were extracted from a deciduous forest soil and inoculated into an aqueous medium. Various N pool dilution/enrichment experiments were carried out to: (1) calculate the gross N immobilization and remineralization rates; (2) investigate their dependence on NH(4)(+)and NO(3)(-) concentrations; (3) establish the microbial preference for NH(4)(+)and NO(3)(-) depending on the NH(4)(+)/NO(3)(-) concentration ratio. Remineralization of microbial N occurred mainly at high growth rates and NH(4)(+) concentrations. There was a positive correlation between NH(4)(+) immobilization and remineralization rates, and intracellular recycling of N seemed to be an efficient way for bacteria to withstand low inorganic N concentrations. Thus, extensive remineralization of microbial N is likely to occur only when environmental conditions promote high growth rates. The results support previous observations of high NO(3)(-) immobilization rates, especially at low NH(4)(+) concentrations, but NO(3)(-) was also immobilized at high NH(4) concentrations. The latter can be understood if part of the microbial community has a preference for NO(3)(-) over NH(4)(+).