The inhibitory effect of different sulfur compounds and antioxidants at the activity level of matrix metalloproteinase (MMP)-2 and -9 during neonatal hypoxia-reoxygenation is unknown. The tissue activity of MMP-2 and -9 was first determined by gelatin zymography in different tissues of 6 newborn piglets that underwent alveolar hypoxia and reoxygenation. The in vitro inhibitory effects of sulfur compounds and antioxidants with or without the thiol group were compared at the highest concentrations of MMP-2 and -9 found. These compounds included: amino acids containing sulfur [cysteine, DL-homocysteine, L-methionine] and not containing sulfur [L-histidine], antioxidants containing sulfur [L-glutathione and N-acetyl-cysteine] and not containing sulfur [ascorbic acid], and oxidized glutathione. Lung had the highest activity of MMP-2 and -9 among the tissues studied. The compounds showed differential effects on the activity of MMP-2 and -9. The order of the potency of inhibition of these compounds for MMP-2 was cysteine> or =histidine> or =ascorbic acid> or =glutathione> or =oxidized glutathione> or =homocysteine> or =N-acetyl-cysteine>methionine, whereas for MMP-9, it was cysteine> or =ascorbic acid> or =histidine>glutathione>homocysteine>N-acetyl-cysteine>oxidized glutathione>methionine. The IC50 values of these compounds on MMP-2 were significantly lower than the corresponding IC50 values on MMP-9. In conclusions, at the activity level of MMP-2 and -9 measured after neonatal hypoxia-reoxygenation, cysteine showed the highest potency of inhibition. The compounds showed different potencies of inhibition, regardless of the presence or absence of the thiol group or the antioxidant property of the compound.