In this work we studied catalase activity in the leaves of 10‑months seedlings of Betula pendula Roth var. pendula and Betula pendula var. carelica (Mercklin). This kind of study was carried out for the first time. We have adapted the spectrophotometric method for the determination of catalase activity for the studied objects. We show how optical density depended on the concentration of the substrate (hydrogen peroxide) and how the enzyme activity depended on the reaction time. We selected the conditions (time, substrate concentration) for the determination of the enzyme activity and modified the formula for catalase activity calculations. The catalase oxidation reaction was observed at different phases of leaf development, which differed one from another in morphometric indices. The division into phases was as follows: leaves 1–2 cm long – phase I, 3–4 cm long – phase II, 5–6 cm long – phase III, 7–8 cm long – phase IV. Information is provided on the dynamics of catalase activity changes for each of the studied birch forms (var. pendula and var. carelica, respectively), as well as in comparison. It was found that catalase activity was significantly higher in var. pendula seedlings than in var. carelica seedlings at all the studied phases. Both forms displayed an increase in catalase activity with increasing leaf length. Thus, the obtained differences have given us the reason to discuss the role of leaves as the test object for detection of anomalies in the wood structure of different birch forms at early stages of ontogeny.