Iron metabolism during lactation in the rabbit.

I . Iron metabolism was studied during lactation in the rabbit by the intravenous injection of 59Fe in order to measure plasma Fe turnover, Fe transfer to the milk and Fe absorption by the suckling young. 2. Although the plasma and red cell volumes were elevated in the lactating animal a significant increase of plasma Fe turnover was not demonstrated. 3 . Between 2.5 and 11.5 yo of the injected dose of 50Fe appeared in the milk during the following 2-3 weeks. Secretion into the milk continued for at least 2 weeks, and the specific activity of Fe was higher in milk than in plasma, suggesting that the mammary gland contained a pool of Fe, with a low turnover rate from which milk Fe was derived. On the average, 1 5 0 3 1 5 ,ug Fe were secreted in the milk per day. 4. Almost all the 50Fe ingested by suckling rabbits, either in milk or sodium chloride solution, was absorbed. 5. It is concluded that in the rabbit the relatively small increase of body Fe which occurs during suckling is entirely due to the limited supply of Fe available in the milk. The high concentration of Fe-binding protein present in rabbit milk did not appear to aid Fe transfer into the milk or to affect Fe absorption by the young.