1. Four groups of 600 juvenile coho salmon (Oncorhynchus kisutch) of 3-8g body weight were exposed to sublethal concentrations of copper acetate in water of hardness 280 mg/l (as CaCO3). 2. Flow-through conditions were maintained for a period of 10 weeks, during which the copper concentrations to which the groups of fish were exposed were approximately 1, 50, 100 and 150 micrograms Cu/l. 3. Hepatic metallothionein concentrations were measured using differential pulse polarography in filtrates of heat-denatured saline extracts of the tissues. 4. These values increased as a function of the copper concentration to maximum levels after 4 weeks of continuous exposure and thereafter remained more or less constant. 5. Flow-through 168 hr LC50 values for fish tested at intervals throughout the experiment increased to maximum values (2.5 X control values) at 1 week and during the next 3 weeks fell somewhat to stable but elevated values. 6. At 4 weeks, these were related to the metallothionein (MT) values by the regression equation 168 hr LC50 = 2.423 (+/- 0.455) X MT + 48.6 (+/- 31.1). 7. The probability that the relationship was not accidental exceeded 97%.