Changes in extracellular peroxidase activity may be mediated by phytochrome as a means of regulating grown rate changes. This was tested in the first internode of light grown mustard seedlings. A correlation between an extracellular anionic peroxidase isoform (A4), extracted by infiltration/centrifugation, was found to decrease in activity by 50% when growth rate was enhanced by a low R:FR ratio. The preparation of protoplasts revealed A4 to be more or less exclusively extracellular. The phytochrome-regulation of A4, apparent in the small percentage of the enzyme that could be extracted by infiltration/centrifugation, was not repeated when the major pool of A4 was examined. A light regulated increase in peroxidase activity was however found in mustard hypocotyls. The light-regulation of cucumber hypocotyl growth was tested and changes in a ionically bound (IB) cationic peroxidase fraction examined. More specifically the use of a long hypocotyl, phytochrome B deficient cucumber mutant (lh), enabled further speculation as to the phytochrome species involved in these changes. When examined, the IB peroxidase activity increased in activity within two hours of the addition of supplementary FR light (a low R:FR ratio), correlating with a change in the rate of growth that could be detected using a mm scale ruler. Whether changes in extracellular peroxidase activity constitute a primary mechanism in the phytochrome-mediation of growth rate changes in light-grown cucumber hypocotyls remained indeterminable. Speculations and possible importance of the observed correlations are discussed. In etiolated cucumber seedlings phytochrome has been shown to control growth within minutes of exposure to light. The extracellular peroxidase activity however, remained unaltered until two days after the commencement of de-etiolation. Thus it was postulated that there appears to be two separate mechanisms in cucumber hypocotyls by which phytochrome-mediated growth rate changes and associated changes in cell wall extensibility are regulated.