We immersed the root systems of three emergent aquatic monocots (Cyperus involucratus Rottb., Eleocharis sphacelata R. Br. (both Cyperaceae), and Juncus ingens N. A. Wakef. (Juncaceae)) in nutrient agar gels with pH indicators and pH microelectrodes to study H+ exchange by root systems. For all three species, the pH changed at the tips of the adventitious roots and around young laterals. Older, lignified surfaces had little effect on pH. The pH changes around the laterals were affected by the form of nitrogen in the nutrient gel When NO3− was supplied as the N source, there was a small pH increase, whereas NH4+ enhanced H+ release, causing the pH adjacent to the laterals to decrease below 4.5. In contrast, root tips acidified the media, irrespective of the nutrient composition. H+ exchange was quantified from titrimetric assays of H+ exchange in nutrient solutions. There was a net H+ efflux from the roots of all three species into complete nutrient solutions with NH4+ as the N source, ranging from 246 μmol H+ h−1 g−1 d. wt for C. involucratus, to 32 μmol H+ h−1 g−1 d. wt for J. ingens. The H+ efflux into similar nutrient solutions with NO3− as the N source ranged from −89 μmol H+ h−1 g−1 d. wt for C. involucratus to −0.8μmol H+ h−1 d. wt for J. ingens. By assuming that H+ exchange was mainly located in the laterals, rates of H+ excretion as high as 542 μmol H+ h−1 g−1 d. wt can be calculated for C. involucratus in NH4+ solutions. Rates of H+ excretion by C. involucratus were scarcely affected by solutions of most Other ions. The NH4+-induced H+ release was dramatically reduced by H+ pump inhibitors, attaining only 18% of control rates with 50 μM diethylstilbestrol and 38% of control rates with 5 μM dicyclohexylcarbodiimide.