The matrix metalloproteinase stromelysin-1 (MMP-3) is inhibited more strongly by peptidyl phosphinic acid 7 than by its corresponding phosphonamidate and phosphonate analogs. Extending a benzyl group at P-1' to a phenylethyl group in 8 further increases the potency (K-i = 1.4 nM). Enhanced potency with an extended substituent into the P-3 region was observed.