Abstract <p>The growth-regulating effect of brassinosteroids (BS) with lactone and ketone structures was studied in relation to their concentration-dependent influence on the root system growth of flax (<i>Linum usitatissimum</i> L<i>.</i>) and spring barley (<i>Hordeum vulgare</i> L<i>.</i>) seedlings under various stress conditions. Data from our laboratory studies, which investigated the growth of fiber flax and spring barley seedlings in the presence of brassinosteroids under chemical stress, were analyzed. Narrow concentration ranges of brassinosteroids (10⁻⁷–10⁻⁸ M) were identified, within which they either weakened or enhanced the inhibitory effect of stressors on the seedlings’ root systems. The concentration ranges of BS that produced the maximum stress-protective effect were nearly identical for both plant species. The features of the anti-stress effect of brassinosteroids on the growth of seedlings were studied, and a hypothesis was proposed regarding their non-genomic pathway in mediating adaptogenic responses.</p>