Four homozygous (HomIso) and eight heterozygous isogenic (HetIso) strains of common carp were used in two separate experiments to examine the genetic background of a stress-related cortisol response due to a rapid temperature decrease. The homozygous sires (two high and two low) and dams (high and low) used to obtain the strains were selected based on their estimated breeding value for this trait at an age of 15 months (EBV15). In both experiments, the strains were subjected to a 9 °C cold shock at an age of 5 months. The ranking in plasma cortisol levels of the HomIso strains was identical to the ranking in EBV15 of the sires and the maximal difference of 350 nmol l-1 was similar to the expected difference based on these EBV15's. Differences between the HetIso strains were smaller than expected, and influence of non-additive genetic effects could not be detected (PDS=0.14). Estimated breeding values based on the performance of the androgenetic progeny (EBV5) in experiment 1 and general combining abilities (GCA) of the sires and dams calculated in experiment 2 were positively correlated with the EBV15 (r not significantly different from 1), providing no evidence that the stress response at 5 and 15 months are different traits. Based on the results of these experiments, it can be argued that the best method to change the stress responsiveness of common carp would be through selective breeding (exploiting additive genetic effects) rather than through crossbreeding (exploiting non-additive genetic effects).