1. A short-term exposure of adult Wistar rats to Cu (50 micrograms/ml) and Cd (10.0 micrograms/ml drinking water) caused significant changes in the subcellular concentrations of Cd, Cu, Zn and metallothionein (MT) in the liver and kidney; the concentrations were close to the physiological values, however. 2. To establish a relationship between these changes in the subcellular concentrations of Cd, Cu, Zn and the level of MT in the post-mitochondrial fraction of the liver and kidney, the analytical data (N = 42) were subjected to the multiple regression analysis. 3. The analysis showed that MT synthesis in the liver was principally induced by small amounts of Cd (0.32-1.4 micrograms/g wet wt) whereas in the kidney a level of MT in the post-mitochondrial fraction correlated positively with the renal Cd and Cu, as well as with the level of this protein in the liver. 4. The above results together with the positive correlation between the level of MT in the post-mitochondrial fraction and the concentration of Cu in this fraction, as well as the fact that under normal physiological conditions the capacity of MT (beta-domain) in the liver and kidney was sufficient to bind 50-100% of the total post-mitochondrial Cu suggest that MT, first induced by small amounts of Cd, may be involved in the metabolism of Cu.