Cultures of Acer pseudoplatantis L. cells have been established in a chemostat with nitrogen as the limiting nutrient factor. During prolonged growth at three different dilutions several enzymes concerned with the assimilation of nitrate and urea from the culture medium achieved steady states of activity. Neither the enzyme activities nor the amino-acid contents of the cells showed marked changes with change in dilution under the nitrogen-limiting conditions employed here. One of the steady states was perturbed by supplementing the culture medium with gluta-mate as an additional nitrogen source. This caused a rapid and considerable enhancement of the alanine content of the cells, presumably resulting from transamination between pyruvate and the incoming glutamate. In the longer term, the transition caused an elevation of the levels of enzymes concerned with the diversification of nitrogen from glutamate (glutamate-oxaloacetate and glutamate-pyruvato transaminases and γ-glutamyl transferase), whilst enzymes concerned with glutamate formation (urease and, most probably, nitrate reductase) declined in activity