The ontogeny of peroxidase activity and isoenzyme pattern was investigated in the stem of dwarf pea plants. Peroxidase activity per unit soluble protein was a given internode is highest in the youngest growth stage, drops during elongation, remains constant upon cessation of growth, and increase at senescence. The lower the internode on the stem the higher is its peroxidase activity. These developmental differences are already apparent at the youngest growth stage of the internodes adn increase during elongation. Several anodic and five cathodic isoperoxidases are apparent after starch gel electrophoresis. This pattern is constant for all internodes at all growth stages, but the relative importance of particular isoenzymes changes with time. Gibberellic acid (GA3) treatment causes greatly elongated internodes, decreased soluble protein, and inhibition of the rise in peroxidase activity within 4–8 h. Application of GA3 to young internodes leads to a persistent depression in peroxidase activity, while treated older internodes suffer only a temporary depression. GA3 causes no qualitative changes in the isoenzyme pattern but produces some quantitative alterations in internodes in which its influence on peroxidase activity is persistent. Decapitation of untreated and GA3-treated dwarfs has little influence on internode elongation, causes an increase in peroxidase activity, especially in the upper internodes, and alters the relative activity of particular isoenzymes. By contrast, decapitation inhibits elongation of young internodes in genetically tall pea plants.