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In this study, the influence of temperature variation on the blood and oxidative stress parameters of pacu Piaractus mesopotamicus juveniles was evaluated. Fish were subjected to gradual changes of temperature, over 5 days, by 24 degrees C until reaching to experimental temperatures 18, 21, 24 (control), 27 and 30 degrees C. There were significant differences (p < .05) in glucose and lactate of blood in both the haematological relationships and oxidative stress parameters. Glucose levels increased at 30 degrees C and decreased at 21 degrees C. Lactate levels were highest at 18 degrees C. While the haematocrit was higher at all altered temperatures, haemoglobin levels decreased at 21 degrees C and increased at 30 degrees C, causing alterations in haematimetric indices. In the liver, glutathione-S-transferase (GST) activity was reduced at 18 degrees C while antioxidant capacity against peroxyl radicals (ACAP) was highest at 30 degrees C. Thiobarbituric acid reactive substances (TBARS) increased at 18, 21 and 27 degrees C. In the muscle, ACAP decreased at 27 and 30 degrees C and TBARS increased at 30 degrees C. The results indicate that the species shows sensitivity to temperature variations. Accordingly, we conclude that with short-term temperature variations >3 degrees C, it is possible to induce higher alterations in the observed parameters in pacu juveniles.