Active Arthus reactions were provoked by injections of 100 micrograms horseradish peroxidase (HRP), 10 micrograms HRP and 100 micrograms bovine serum albumin (BSA) into the skin of sensitized guinea pigs. Labeling indices (LI) of epidermal basal cells were measured 1, 4, 8, 24, 48 and 72 h later by the in vivo 3H-thymidine labeling technique, and compared with those obtained with injections of antigens into the skin of non-sensitized guinea pigs. From 1-8 h after the induction of an active Arthus reaction, the LI of epidermal basal cells of the skin injected with 100 micrograms HRP decreased to a remarkably low value. On the other hand, those obtained with the reaction against 10 micrograms HRP were significantly high. At 24 h after the reaction, LI were as high as those obtained in non-sensitized guinea pigs with control intradermal injections, though the former persisted high until 48 h after the injection. In addition, decreased LI of the epidermal basal cells were observed in the skin 4 h after intradermal injections of immune complexes. It was suggested that DNA synthetic activity of the epidermis increases in a mild active Arthus reaction, while the activity may be suppressed in a severe active Arthus reaction up to 8 h after provocation.