The cycling of cadmium and mercury between substrate and fruiting bodies in a model system with wood colonizing basidiomycete Agrocybe aegerita 109 203 was studied. When radiolabeled CdC12 and HgC12 were applied to the fruiting bodies of the first flush, they were translocated via substrate into successive harvests. Cadmium and mercury displayed different patterns of distribution in the system. On a percent basis, more cadmium went from the fruiting bodies into the substrate and was retained there. Only minor portions of the metal were translocated into consecutive crops. In contrast, more mercury was retained in the treated fruiting bodies. The fraction which had penetrated into the substrate, however, was more easily translocated into fruiting bodies of successive crops. When calculated on a dry weight basis, the amount of both metals decreased in consecutive harvests. At the end of the experiment, in following distribution patterns for cadmium and mercury ,were observed: Cd 2+ : first crop (treated), 9.5 %; substrate, 77% ; combined successive crops (untreated), 9.5%; Hg2+: first crop (treated), 36.5%i substrate 21.5%; combined successive crops (untreated), 37%. The patterns reveal that mercury is more mobile in the substrate and therefore more easily translocated to successive fruiting body generations. Hence, from a nutritional point of view, mercury would seem to be more hazardous than cadmium.