Ornithine decarboxylase (ODC), antizyme (AZ), and antizyme inhibitor (AIn) play a key role in regulation of intracellular polyamine levels by forming a regulatory circuit through their interactions. To gain insight into their functional importance in cell growth and differentiation, we systematically examined the changes of their expression, cellular polyamine contents, expression of genes related to polyamine metabolism, and beta-casein gene expression during murine mammary gland development. The activity of ODC and AZ1 as well as putrescine level were low in the virgin and involuting stages, but they increased markedly during late pregnancy and early lactation when mammary cells proliferate extensively and begin to augment their differentiated function. The level of spermidine and expression of genes encoding spermidine synthase and AIn increased in a closely parallel manner with that of casein gene expression during pregnancy and lactation. On the other hand, the level of spermidine/spermine N(1)-acetyltransferase (SSAT) mRNA and AZ2 mRNA decreased during those periods. Immunohistochemical analysis showed the translocation of ODC and AIn between the nucleus and cytoplasm and the continuous presence of AZ in the nucleus during gland development. Reduction of AIn by RNA interference inhibited expression of beta-casein gene stimulated by lactogenic hormones in HC11 cells. In contrast, reduction of AZ by AZsiRNA resulted in the small increase of beta-casein gene expression. These results suggested that AIn plays an important role in the mammary gland development by changing its expression, subcellular localization, and functional interplay with AZ.