The polyamines are normal cell constituents considered to have an important role in the regulation of proliferation and differentiation. DFMO is an irreversible, enzyme-activated, suicide inhibitor of ornithine decarboxylase (ODC), the enzyme responsible for the first and rate-limiting step in mammalian polyamine synthesis. Preliminary data show that DFMO inhibits tumor cell growth in vitro and in vivo, and that it demonstrates chemopreventive activity in a variety of animal tumors. The prostate contains some of the highest concentrations of polyamines and of polyamine-synthetic enzymes (including ODC) in the mammalian organism. ODC activity in the prostate was shown to be more susceptible to DFMO inhibition than in other organs. We have found the ODC activity of the Dunning R3327 rat prostatic carcinomas to be as sensitive to inhibition by DFMO as the normal rat prostate. Furthermore, DFMO was inhibitory to the growth of the tumor both in vitro and in vivo. Given the slow growth rate and long latency period of human prostate cancer and the preliminary DFMO data, we suggest that clinical trials to evaluate the chemopreventive potential of DFMO in prostatic carcinoma deserve serious consideration.