The effects of juglone, an allelochemical, on cell elongation, respiration, the content of protein and α-amylase activity induced by gibberellin in aleurone cells of barley seeds, and its interactions with SH groups in amino acid, peptide, protein, and plant extracts were studied. Juglone inhibited the cell elongation in epicotyls of etiolated bean seedlings by 50 and 80%, and inhibited the elongation of coleoptiles of etiolated barley seedlings by 75 and 96% at 10^-4M and 10^-3M, respectively. The inhibition of the elongation of root cells of bean and barley were stronger. 10^-3 M of juglone solution decreased the respiration (O₂ uptake) of roots of bean and lettuce to a significant degree 10 and 30min after the beginning of treatment and reduced the O₂ uptake to 0 after 2 and 6hrs treatment, respectively. It de creased the content of total soluble protein and α-amylase activity induced by gibberellin by 74% and 78%, respectively, in the aleurone cells of barley embryoless half seeds at 10^-3M. Juglone at 10^-3M could completely react with the SH groups in cysteine, glutathione, bovine serum albumin and decreased the contents of SH groups in crude extracts from germinated barley seeds and epicotyls of etiolated seedlings of bean by 60 and 55%, respectively. These results suggest that juglone may be a metabolic inhibitor which prevents many (if not all) physiological and biochemical processes involving in the function of SH groups by interfering with the SH-group bearing substances including amino acids, peptides and enzymes.