The activity of uterine ornithine decarboxylase (ODC) was measured during the 24 hours after systemic induction of decidualization. Following a latent period of 2.5 hours, activity rose and reached a peak of 5.6 +/- 1.1 fold at 5 hours after induction. The activity then declined and a second, lower peak was seen at 21 hours. The increase in ODC activity was suppressed by treatment with cycloheximide (50 mg/kg) and dactinomycin, 500 microgram/rat, confirming that the enzyme response reflects protein synthesis. The increase and the base-line level of ODC activity were suppressed by ergocornine pretreatment (1 mg/rat) which disturbs the endocrine balance of pseudopregnancy. It was concluded that the growth and differentiation of decidual tissue began 2.5 hours after application of the induction stimulus.