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Wheat (Triticum aestivum L.) is one of the most significant food cereal in the world. Under natural conditions, biotic and abiotic stress factors can seriously endanger the plant growth and development. Fusarium head blight (FHB), mainly caused by F. graminearum and F. culmorum, is a disease that has negative effects on economy, namely on the yield and the quality of the grain. In this research, the activities of guaiacol peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX) and polyphenol oxidase (PPO), as well as H2O2 concentration and malonedialdehyde (MDA) content, were determined in three wheat genotypes of various resistance to FHB ("Super Žitarka"- sensitive; "Lucija"- moderately resistant, and "Apache"- resistant) after the 2nd and the 4th day of infestation. The infected plants of "Lucija" showed higher POD activity compared to control after the 4th day, and increased POD activity compared to the 2nd day of infection. On the 2nd day, "Apache" responded to pathogen by the enhanced APX activity, and higher MDA level, compared to the 4th day of treatment when it started to decrease. Moreover, the treated plants of the same genotype showed the increased MDA level on the 2nd day, unlike the untreated plants. Contrarily, H2O2 concentration in "Apache" showed the tendency to increase with the time of Fusarium-exposure. In comparison to the 2nd day of treatment, PPO activity decreased in "Super Žitarka" on the 4th day. In all genotypes at both sampling points, CAT activity did not change significantly under the pathogen attack. Overproduction of H2O2 accompanied with the increased APX activity 2 days after infection may lead to the conclusion that FHB tolerance of "Apache" is ensured by the earlier activation of the defence mechanisms. [ABSTRACT FROM AUTHOR]