Intraperitoneal injection of the non-phorbol tumor promoter anthralin (1,8-dihydroxy-9-anthrone) in male rats resulted in an increase of hepatic ornithine decarboxylase (ODC) activity. Maximal activity was observed 8 h after promoter administration reaching levels about 30 times over control. The kinetics of anthralin dependent ODC induction differed markedly from that by either 12-O-tetradecanoylphorbol-13-acetate (TPA) or phenobarbital (PB) (Bisschop et al., Carcinogenesis 2 (1981) 1282). With anthralin a slow decrease of ODC back to control level is observed approximately within 22 h. In contrast, ODC induction mediated by other tumor promoters like TPA and PB decreased to control levels within 4-6 hours. Administration of a second dose of anthralin 8 h after the first dose prevented the activity decrease as normally observed after a single dose of a tumor promoter. This effect lasted at least 10 h. ODC activity induction occurred in a dose-dependent manner being linear from 10-2000 micrograms anthralin/kg body wt. Pretreatment of the animals either with actinomycin D or with cycloheximide completely blocked anthralin mediated ODC induction suggesting that de novo ODC-mRNA synthesis and subsequent translation is involved in this process.