Ornithine decarboxylase from Trypanosoma brucei brucei was inhibited by several substrate (ornithine) and product (putrescine) analogs both in vitro and in vivo. Since alpha-difluoromethylornithine is effective for the treatment of experimental and clinical African trypanosomiasis, it was possible that the more potent ornithine and putrescine analogs might be more active in treating the disease. However, only alpha-monofluoromethyldehydroornithine methyl ester was more potent than alpha-difluromethylornithine against mouse trypanosomiasis and warrants further study in model infections.