Growth patterns and mean proton efflux rates (E H+) in roots of rice seedlings (Oryza sativa cv. IAC 47) grown under light conditions in a greenhouse environment were studied. Fifteen days after emergence, seedlings were transferred to pots containing 3.7 L of nutrient solution with 1.43 mmol NNH4+ L¹ as the sole source of nitrogen, to induce four sequential cycles of solution acidification, starting with a pH value of 5.3. Mean proton effluxes were computed based on growth periods of 48 h (from 0 to 192 h). E H+ values were estimated from values of relative root growth rates (RGR), expressed as root area (RA) or fresh mass (FM), and the linear coefficient of the regression of total free protons in nutrient solution (U H+) on RA or FM. Total free protons in the solution and the acidification gradients (deltaU H+/deltaMF) were better related to FM than to RA. Root growth rates were affected by photosynthetic irradiance and temperature fluctuations in the greenhouse, and were not clearly related to E H+ values. However, it was concluded that when a linear relation between U H+ and fresh root mass is found, E H+ estimates expressed by FM may be more reliable than when expressed by RA units.