Ornithine decarboxylase activity and polysome patterns of the rat uterus were studied during the normal estrous cycle. Specific and per cell activity of ornithine decarboxylase at diestrus I was almost nil. The activity increases notably at diestrus II (22 nmoles/mg DNA/hr) and peaks at pro—estrus (98 nmoles/mg DNA/ hr). Then the activity decreases as rapidly as it increased, being possible to postulate that synthesis of this enzyme stops at some time during oestrus. The increase in ornithine decarboxylase activity is accompanied by an increase in the polyamine pools, putrescine concentration tripled and spermine and spermidine concentrations doubled from diestrus II up to pro—estrus. The concentration of the polyamines was kept constant during estrus and thereafter gradually decreased, reaching the lowest levels at diestrus II. The polysome pattern exhibits drastic changes that can be correlated with the stabilizing role of the polyamines. Diestrus II shows a clear absence of heavy polysomes. This behavior contrasts with that observed at pro—estrus, when heavy polysomes of up to 8 ribosomal units were observed. It is concluded that in the normal cycling rats the changes found in the studied parameters parallel the changes that may be induced in the laboratory animal by the external administration of hormones. (Endocrinology92: 1107, 1973)