The avoidance response of rainbow trout Salmo gairdneri to copper and nickel solutions under several exposure regimes was evaluated with a computer-based data aquisition system interfaced with a linear, plexiglass chamber with countercurrent water flow. The best estimates of the avoidance thresholds for these metals were 6.4 micrograms/1 total copper and 23.9 micrograms/1 total nickel (95% confidence limits were 2.6-15.5 and 10.2-54.3 micrograms/1, respectively). For each metal, avoidance threshold concentrations were different whether fish were exposed in shallow or steep concentration gradients. Also, no significant differences were found when fish received multiple, increasing exposures, or a single exposure in steep-gradient conditions. Rainbow trout initially avoided low copper concentrations, but were attracted to higher ones (330-390 micrograms/1) during shallow-gradient tests. Under similar test conditions, fish were attracted to low nickel concentrations (about 6 micrograms/1) but avoided higher levels ( >19 micrograms/1).