The effects of an oligogalacturonic acid (OGA) pool on root length of intact alfalfa seedlings (Medicago sativa L.), on extracellular pH and on both extracellular and intracellular O dynamics were examined in this study. Lower OGA concentrations (25, 50 and 75μgmL) promoted root length, but 50μgmL had a stronger effect in promoting growth, while the higher OGA concentration (100μgmL) had no significant effect. Extracellular alkalinization was tested only at concentrations higher than 50μgmL OGA, showing that the response is determined not only by the specific size of OGA, but also by the concentration of OGA. The promoting effect of OGA on root growth at 25, 50 and 75μgmL OGA concentrations in alfalfa root appeared to be unrelated to extracellular alkalinization. A possible explanation could be the induction of an O burst at non-toxic levels, which could drive directly or indirectly several processes associated with root elongation in 25, 50 and 75μgmL OGA-treated seedlings. Analyses using confocal microscopy showed that the increase in the O generation, mainly in the epidermal cells, induced by 50μgmL OGA could be related to the promoting effect on root growth. The combination of OGA with DPI allowed us to demonstrate that there are different O-generating sources in the epidermal cells of the meristematic zone, likely NADPH oxidase and oxidases or oxido-reductase enzymes, insensitive to DPI, that maintain detectable O accumulation at 60 and 120min of treatment. These results suggest that OGA induce an oxidative burst by several O-generating sources in the active growth zones.