15N was used to give evidence of NH3 and N2O absorption by maize leaves. A short-term labelling method (90 min) was chosen with high NH3 concentration (40 μ1 1−1) to assess uptake mechanisms. Two initial entry pathways were suggested for NH3: fast uptake with immediate metabolism and fast storage with progressive metabolism. A storage compartment delayed NH3 absorption after exposure. Similar mechanisms might be involved in N2O uptake.