The effect of phenytoin on the mitotic activity of gingival tissue as well as cultured mammalian fibroblasts was studied. This was measured quantitatively using autoradiography utilizing tritiated thymidine. Hyperplastic gingiva removed from patients treated with phenytoin and subjected to autoradiography showed a marked increase in the mitotic activity of gingival cells. This was also the case with cultured fibroblasts. The mitotic index was higher in the treated fibroblasts than that of the control. Ascorbic acid added to the cultures markedly reduced the mitotic activity of these cells. These results agree with other studies that phenytoin may be a factor in direct stimulation of the mitotic activity and gingival hyperplasia seen in patients taking this medication.