The activities of ornithine decarboxylase (ODC) and S-adenosyl-L-methionine decarboxylase (SAMDC) and the concentrations of putrescine, spermidine and spermine were measured in mouse uterus placenta and foetus during gestation. The prominent post-implantation biochemical changes in the intact uterus were associated mainly with the deciduomata and significant ODC activity was located in the embryo. Administration of the irreversible inhibitor of ODC, alpha-difluoromethylornithine, DFMO, 2% inthe drinking water during days 5-8 of gestation, abolished the inareases in uterine ODC activity, putrescine and spermidine concentrations and enhanced the activity of SAMDC. Treated animals showed no signs of pregnancy when autopsied on day 18. The alterations in deciduomal weight and the changes in uterine DNA, RNA and protein content indicated that decidualization following DFMO took place normaly but that embryonic growth was arrested. Treatment on single days with DFMO, 200 mg/kg every six h, revealed optimal contragestational effects on day 8 which corresponded exactly to the time of the peak in deciduomal ODC activity. Treatment with DFMO at times other than during the vulnerable period of days 5-8 has less prominent effects on gestation. An increase in ODC activity appears to be an essential factor during a short, but critical, period after implantation for continued murine embryonal growth.