Prolidase [E.C. 3.4.13.9] is a cytosolic enzyme which specifically splits imidodipeptides with C-terminal proline or hydroxyproline. This enzyme plays an important role in the recycling of proline for collagen biosynthesis and cell growth. Prolidase activity was measured in cultured human skin fibroblasts, treated with some non-steroid antiinflammatory drugs (acetyl-salicylic acid, sodium salicylate, phenylbutazone, indometacin). It was found that 24 h treatment of confluent cells with the examined drugs induced significant, dose dependent decrease in the fibroblast's prolidase activity. Simultaneously, the antiinflammatory drugs inhibited collagen biosynthesis and decreased extracellular content of this protein in extracellular matrix produced by fibroblasts. These observations suggest that non-steroid antiinflammatory drugs affect the metabolism of collagen through inhibition of prolidase activity in the collagen synthesizing cells.