Class III peroxidases (EC 1.11.1.7) have use a wide range of substrates and perform numerous functions, including synthesis of monolignols, lignin precursors. The activity and tissue localization of cationic guaiacol (GPOX) and anionic benzidine peroxidases (BPOX) were studied in the first internode of zinnia plants of different ages. The lignin in cross sections of plant tissues was stained by phloroglucinol-HClTthe lignin content was determined by Klason. Enzyme activity and the H2O2 amount were determined spectrophotometrically. The hypocotyl grew for 40 days and the lignin content for 60 days. In 20-days-old plants, lignin was detected in protoxylem, and in 60-days-old—in sclerenchyma, protoxylem, and metaxylem. Enzyme histochemistry revealed that BPOX was localized in endoderm, phloem, and protoxylem, while GPOX—in the metaxylem and sclerenchyma. A moderate increase in GPOX activity during internode growth was shown. In contrast, BPOX activity was high at the initial growth stage, and declined to the 60th day. Thus, the most intense lignification in mechanical tissue and xylem occurred during the period from 20 to 40 days of plant growth. BPOX is likely involved in the process at the early stages of growth, while GPOX is responsible for sclerenchyma and metaxylem lignification at the later stages.