Smoltifying Atlantic salmon were treated for 6 weeks in freshwater with 0, 200, and 400 ppb zinc (as Zn SO4). After 6 weeks salmon were transferred to a salt water seapen and exposed to infection with Lepeophtheirus salmonis for 14 weeks. Zinc treatment resulted in some physiological changes consistent with increased stress, such as decreased leukocrit values, and increased plasma cortisol levels. Plasma glucose levels were significantly elevated in fish previously treated with zinc, but this was only evident after fish had been in a seapen, 14 weeks subsequent to treatment. Improved conversion of tetraiodothyronine to triiodothyronine, as well as no changes in interrenal cell nuclear diameters suggested that overall stress effects were low. Fish exposed to 200 ppb and 400 ppb zinc showed gill pathology. Infection intensity with L. salmonis was significantly higher on salmon previously exposed to 400 ppb zinc. This research suggests that for smoltifying Atlantic salmon, the No Observed Pathological Effect (NOPE) level for exposure for ZNSO4 may be around 150 ppb.