Blood was removed from ten adult rainbow trout (Salmo gairdneri) on a sequential daily (7 days), weekly (4 weeks), and monthly (1 month) schedule and analyzed for hematocrit, plasma protein, acid phosphatase (AP), lactic dehydrogenase (LDH), and creatine phosphokinase (CPK). Of the parameters examined, hematocrit, lactic dehydrogenase, and creatine phosphokinase were found to be the most significant indicators of stress due to repeated sampling. We showed that, under conditions similar to those of this investigation, daily and weekly sampling of approximately 0.2% of the body weight in blood produced decidedly deleterious effects of the test organisms. Starvation was a secondary stressor and is believed to have had an influence on blood characteristics as the study entered its later phases.