The effect of two antioxidants on the activities of matrix metalloproteinases (MMP) secreted by normal (3T3) and transformed (3T3-SV40) mouse fibroblasts was examined. We compared the effect of N-acetylcystein (NAC) and alpha-lipoic acid (ALA) on two gelatinases, MMP-2 and MMP-9. Gel zymography demonstrated that activity of MMP-2 was higher in normal 3T3 cells, and MMP-9 activity was higher in transformed 3T3-SV40 cells. NAC action for 2-6 hours completely inhibited MMP-2 and MMP-9 activity in both cell lines. The inhibitory effect almost did not depend on NAC concentration at the range of 1-10 mM. ALA (1.2 mM) affected the cells not so dramatically. ALA decreased the MMP-2 activity in both cellular types. As to MMP-9 activity, it decreased in 3T3 cells and slightly increased in 3T3-SV40 cells in the presence of ALA. The activity of membrane bound and intracellular MMP was not changed under the same conditions. In conclusion, an altered activity of MMP in the presence of an antioxidant may influence the intracellular signalling and cell functions.