Localization of two isoforms of glutamine synthetase (GS; EC 6.3.1.2) was investigated in different cell types, mesophyll cells and bundle sheath cells, of corn (Zea maysL. var. W64A × W182E) leaves by using ion exchange chrotnatography. In whole leaf extracts, relative activities of GS1 (cytosolic GS) and GS2 (chloroplastic GS) were almost equal. Purified mesophyll protoplasts and bundle sheath strands also showed similar proportions of GS1 and GS2. Methionine sulfoximine (1mM) enhanced the accumulation of ammonia when mesophyll protoplasts were incubated with nitrite or when bundle sheath strands were incubated with glycine. This clearly indicates a spatial separation of metabolism of NH+4derived from photorespiration and from reduction of NOJ