This study examined the hematological profiles and responses to exhaustive exercise in 'all-fish' growth hormone (GH) transgenic common carp (Cyprinus carpio L) compared to non-transgenic controls. Transgenic fish had significantly smaller erythrocytes (length, width, surface area and volume), with a larger surface to volume ratios than non-transgenics. There were no significant differences in hematocrit, blood hemoglobin concentrations, plasma glucose, lactate, cholesterol and protein concentrations between the two strains at rest, but the transgenic fish had significantly lower resting mean corpuscular hemoglobin concentrations (MCHC) or higher plasma triglyceride concentrations than controls. Exhaustive exercise did not induce significant changes in hematocrit, hemoglobin concentrations and MCHC in either strain. The patterns of elevation and recovery in plasma glucose and lactate were the same for transgenic and control fish, although lactate levels were lower in the transgnic fish than controls at 1 h and 2 h post-exercise. There was a significant elevation in plasma cholesterol and triglyceride concentrations for 1 h post-exercise in transgenics but not in non-transgenic controls. Plasma protein concentrations were not altered by exhaustive exercise in transgenics but were lower from 2 h post-exercise in non-transgenic controls. These findings suggest that the 'all-fish' GH transgene significantly affects the erythrocyte morphology and the patterns in hematological response to exhaustive exercise of common carp, especially for lipid metabolism. The greater reliance on triglycerides and a greater erythrocyte surface to volume ratio may help transgenic carp to recover from exhaustive exercise, but further research on muscle metabolism and aerobic metabolic rate are needed in order to confirm this conclusion. (C) 2010 Elsevier B.V. All rights reserved.