In the course of a series of experiments on factorial Fe and Cu supply (0, 25, 250 and 625 micrograms Fe/g; 0, 10, 100 and 250 micrograms Cu/g), t effects on Fe and Cu contents of liver, spleen, kidney, and heart of growing rats were examined. Total contents as well as concentrations in these organs depended not only on the supply with the respective trace element but also on interactions between both trace elements. Iron doses of 250 and 625 micrograms/g resulted in an accumulation of Fe in the liver of Cu-deficient animals, but in reduced Fe contents in spleen, kidney, and heart. Following supplements of 250 micrograms Cu/g diet, liver Fe contents were significantly decreased under these conditions, whereas those of the other organs were not changed at all. On the other side, Cu doses of 100 micrograms/g and more caused a significant increase of liver Cu contents, if Fe supply was deficient or, partially, suboptimal. Total Cu of spleen and kidney and, as a rule, in heart, too, was significantly reduced in these animals. Variations in the retention behaviour of the different organs were observed concerning the interrelationships of the concentrations of Fe and Cu. A strongly inverse reaction of the two elements was observed in the liver of Cu-deficient as well as in sufficiently or excessively Cu-supplied rats. Both Fe and Cu concentrations in kidney and heart, however, increased when the Cu supplement was 10 micrograms/g or more. Cu deficiency caused reduced heart values, whereas in kidney a minimal increase was still observed. In spleen, no interrelationship in any direction was found.