Fish hematological changes during osmotic and cold stress are used to introduce the physiological reactions of the animal to an acute stress. Brook char (Salvelinus fontinalis) were subjected to 1 h of stress before being anesthetized and having blood taken from their caudal vein. Glucose, hemoglobin, hematocrit, and osmolarity were determined in the blood samples. Analyses showed that glucose concentration tends to increase and hematocrit tends to decrease in stressed fish. Changes in hemoglobin concentration occurred only in cold-stressed fish. A rise in blood glucose concentration is the result of cortisol secreted by the hypothalamic-pituitary-adrenal axis. The glucose produced is used as an osmolyte or energy source to resist or combat the stress. In stressed fish, changes in hematocrit could be the result of the osmoconcentration of the blood plasma, as shown by the increase in osmolarity for the same group. In cold-stressed fish, a decrease in hemoglobin concentration could be the result of hemodilution by body cell water. ADV PHYSIOL EDUC 23: 18-23, 2000.