We have demonstrated that the zinc (Zn) concentration of mouse milk declines significantly over the lactation period. Pups radiolabeled in utero with 65Zn were forward-fostered (FF) to nonradiolabeled dams at a later stage of lactation to study the effects of early milk deprivation. Other groups of radiolabeled pups were back-fostered at 5, 10 and 15 days of age to a nonradiolabeled dam who had just given birth; this provided additional colostrum during the suckling period. Litters fostered at birth to an unlabeled dam at d 0 of lactation were used as controls. Weight gain of FF litters decreased and signs of Zn deficiency increased as the foster dam's days of lactation increased. The Zn concentration of kidney, brain and plasma tended to decrease with increasing lactation days of the foster dam. Tibia Zn concentration declined progressively as the lactation days of the foster dam increased, and the concentration of calcium also was lower in all three groups of FF litters than in controls, indicating that bone calcification may have been impaired. Kinetic data proved to be a more sensitive index of Zn status than tissue Zn concentration. The biological half-life of whole-body 65Zn for FF suckling mice increased in a linear fashion with increasing lactation days of the foster dam; whole-body retention of 65Zn for back-fostered litters did not differ from that for controls. Brain, small intestine, kidney muscle, plasma and tibia of pups FF to late-lactating dams seemed to have greater retention of Zn than did controls. Thus, deprivation of early milk impaired growth and development of the mouse neonate despite some ability to conserve Zn.