The purpose of the present study was to compare protein and matrix molecular (collagen and fibronectin) metabolism among fibroblasts cultured from two types of hyperplastic gingiva and normal tissues. Cell cultures were obtained from tissue explants of normal gingiva, the periodontal ligament (PDL), and phenytoin-induced and idiopathic fibromatotic gingiva. The synthetic capacity of matrix proteins of each isolated fibroblast type was studied by metabolic labeling (3H-proline), biochemical purification and characterization. Our results showed that fibroblasts derived from phenytoin-induced gingival hyperplasia synthesized a greater amount of total protein and collagen than the rest of the cell types, based on both the amount of total protein and protein/micrograms DNA. In contrast, fibroblasts obtained from idiopathic gingival fibromatosis exhibited a similar potential for both protein and collagen synthesis as normal gingival and PDL fibroblasts. Normal PDL fibroblasts displayed a significantly higher degree of collagen synthesis than normal gingival fibroblasts; however, no difference in protein production between them was noted. A few variations in fibronectin synthesis/microgram DNA among various fibroblasts from normal and hyperplastic tissues were noted. PDL fibroblasts seemed to produce a greater amount of fibronectin than all of the other cell types. Interestingly, fibroblasts obtained from idiopathic gingival fibromatosis showed a higher DNA content than other fibroblasts, although a decrease in synthesized fibronectin/microgram DNA was noted. In conclusion, the pathogenic mechanism of phenytoin-induced hyperplasia and idiopathic gingival fibromatosis may be different due to their distinct profile of cell proliferation and matrix protein synthesis.