The apoplastic pH of intact green leaves of Bromus erectus was measured non-invasively by inserting blunt microelectrodes through stomatal openings. After making electrical contact, the recorded signal was stable for hours, yielding a pH of 4.67±0.10. The leaves responded to ‘light-off’ with an initial transient acidification and subsequent sustained alkalinization of 0.2–0.3 pH; ‘light-on’ caused the opposite response. Flushing the leaves with 280 nmol NH3 mol−1 air within 18±6 s alkalinized the apoplast by 0.22±0.07 pH, followed by a slower pH increase to reach a steady-state alkalinization of 0.53±0.14 after 19±7 min. This pH shift was persistent as long as the NH3 was flushed, and readily returned to its initial value after replacing the NH3 with clean air. The resultant [NH4+] increase within the apoplast was measured with a NH4+-selective microelectrode. In the presence of 280 nmol NH3 mol−1 air, apoplastic NH4+ initially increased within 15±10 s to 1.53±0.41 mM, to reach a steady state of 1.62±0.16 mM after 27±7 min. An apoplastic buffer capacity of 6 mM pH−1 unit was calculated from the initial changes of pH and [NH4+ ], whereas the steady-state values yielded 2.7 mM pH−1. Infiltrated leaves responded to NH4+ with concentration-dependent depolarizations, the maxima of which yielded saturation kinetics indicating carrier-mediated NH4+ uptake into adjacent cells, as well as a linear component indicating non-specific transport. We infer that the initial alkalinization is due to rapid conversion of NH3 to NH4+, whereas the slower pH increase would be caused by regulatory processes involving both membrane transport, and (mainly) NH4+ assimilation. Possible consequences of the NH3-induced pH shift for the development of plants growing in polluted areas are discussed.