growth, phenolic content, and antioxidant activities of horehound (Marrubium vulgare L.). The long-term salt treatment (up to 100 mM NaCl) resulted in significant (p<0.05) reduction of height, fresh weight, total chlorophyll, and total phenol contents. The negative effect of NaCl was accompanied by a significant restriction in K⁺, Ca²⁺, Fe²⁺, and Zn²⁺ ion uptake, and by an increase in Na⁺ ion concentrations – the effects of which were most pronounced at the highest NaCl level. The content of the main bioactive compound marrubiin decreased with increased NaCl concentrations. The antioxidant activity of the methanol extracts from untreated and salt-treated plants revealed that the extracts from a salt-treated plant with 100 mM exhibits the strongest activity in the DPPH and β-carotene bleaching assays, while it showed no reducing power. The present results suggest that salt treatment negatively affects the morphological, physiological, and biochemical traits of M. vulgare, which appears to be highly sensitive to salinity.