To evaluate the role of ornithine decarboxylase (ODC) and the polyamines in tissue growth and development, neonatal rats were given daily injections of alpha-difluoromethylornithine, a specific, irreversible inhibitor of ODC. Enzyme activity in brain, heart and kidney was effectively inhibited, leading to prompt reductions in putrescine levels which were apparent throughout the 4-week period of drug treatment. Deficits in spermidine levels appeared within several days and remained significant in all three tissues, although some recovery toward control levels was apparent after 2 weeks postnatally. Spermine levels were not decreased and in some cases were actually increased during the course of alpha-difluoromethylornithine administration; assessment of total organ content of spermine or total polyamines per organ (putrescine + spermidine + spermine) indicated that the tissues were still actively increasing their net polyamine content despite continued inhibition of ODC. Growth of the kidney and brain were affected within several days of commencing alpha-difluoromethylornithine treatment, well before the onset of body weight or heart weight deficits. By 14 days of age and thereafter, animals displayed delayed eye-opening, deficient fur growth and shorter body length. These data suggest that the ODC/polyamine system does serve as a modulator of tissue growth during neonatal mammalian development and that differences exist among various tissues in the degree and time course of dependence of growth on polyamines, particularly putrescine and/or spermidine.