Seven blood characteristics — red blood cell count (RBC), hematocrit (Hc), hemoglobin (Hb), plasma chloride (Cl), plasma glutamic oxalacetic transaminase (PGOT, L-aspartate:2-oxoglutarate amino transferase), osmolarity (Os), and total protein (TP) — were measured in brook trout (Salvelinus fontinalis) that had been exposed to three concentrations (67.5–69.2, 38.2–39.0, and 22.8–24.0 μg/liter) of Cu(II) for 6 and 21 days. Concentrations of 67.5–69.2 and 38.2–39.0 μg/liter caused statistically significant increases in RBC, Hc (6-day only), Hb, PGOT, and TP (6-day only), whereas Cl and Os decreased during both exposure periods. Five blood characteristics (RBC, Hb, Hc, Cl, and PGOT) were also measured in brook trout, which were exposed for 337 days to Cu(II) concentrations of 32.5, 17.4, 9.5, 5.7, and 3.4 μg/liter. After this long-term exposure, no changes were observed in the blood except for a measurable decrease in PGOT values at 32.5 and 17.4 μg/liter. The disappearance of initial blood changes, after extended exposure, suggests the transient nature of these early responses. Application of this study to the evaluation of the physical condition of fish and the possible long-range forecasting of reproductive success and survival of a species is discussed.