Light microscopic autoradiography and quantitative tissue analyses were used to examine the distribution of radioiron in larvae (ammocoetes) of the lamprey Petromyzon marinus L. over time following the injection of 55ferrous citrate (55Fe). It was found that a single intraperitoneal injection of 55Fe is an efficient method of introducing iron into ammocoetes, and radioiron retention within the body remained high throughout the 28-d experiment. Immediately after radioiron administration the isotope was transported in the serum fraction of the blood to sites of iron storage and elimination. The liver is the most important target organ for iron in ammocoetes, and radioiron acquired from the alimentary canal, skin, carcass, and blood contributed to the initial increase in this organ. Data indicated only temporary storage in the liver; between 3 and 16 d, radioiron was redistributed by the blood to the skin, kidney, alimentary canal, and carcass. Biliary transport of radioiron from the liver to the lumen of the anterior intestine may also explain the lowering of iron concentrations in the liver and the rise in levels in the alimentary canal during this interval. The redeposition of 55Fe in the liver at 16 d may signify longer-term storage of the metal in this organ. There is movement of radioiron in and out of the carcass tissues over time, and most of the radioactivity is localized in the adipose tissue. The epithelial cells of the proximal tubules of the kidney absorb and concentrate the radioiron presumably present in the glomerular filtrate. This concentration probably reflects both the absorption of iron bound to ferritin and a mechanism necessary for maintaining the high serum iron levels of ammocoetes. The release of mucus from iron-laden mucous cells of the skin and exfoliation of radioiron-laden epithelial cells in specific regions of the posterior intestine are possible routes for iron elimination. However, percent incorporation of the injected radioactivity was still high at the end of the experiment, indicating that iron excretion in ammocoetes is relatively low or that it is a slow process. These data support the notion that lampreys provide a useful animal model for the study of iron metabolism in vertebrates.