The concentration of iron in all mammalian milks falls during lactation while the infant's iron requirement increases. Little is known, however, about the entry of iron into milk. Recently, transferrin receptors have been identified on lactating rat mammary plasma membranes, which may regulate iron entry into mammary tissue, potentiating its availability for subsequent transport into milk. This study was conducted to determine what relationship exists between the declining concentration of milk iron and the transferrin receptor concentration during various stages of lactation. Minimal transferrin receptors were detected in nulliparous rats. Total mammary transferrin receptor content increased during early and mid-lactation while milk iron concentration decreased. The continued appearance of high levels of transferrin receptors throughout lactation, without a concomitant increase in milk iron concentration, suggests a need for iron for functions other than cellular growth or secretion into milk to meet infant needs.