Sexual differences in intestinal iron absorption in Shaver and New Hampshire chickens strains was analyzed with an in vivo perfusion technique in closed circuit. In 6-week-old Shaver chickens, a significantly higher rate was found in females.A comparison of the absorption rates between adult males and laying and nonlaying hens from the New Hampshire strain, showed a significant increase of the net transfer during the laying period. However, we did not find any differences between males and nonlaying hens. From our data, a thicker intestinal wall seemed evident in the adult females, with highest values during laying.Estrogen administration in male chickens increased the plasma iron and produced a defensive reaction to iron absorption. However, during the laying stage, high plasma levels were found to be tied to higher absorption rates.Iron storage in females is higher in proportion to the increase of iron absorption, which is enhanced during the laying period. Estrogen appears to mediate this process, but from present data the point of action has not yet been identified.