Inward rectifier potassium channels (Kir2.x) in cardiac myocytes have an essential role in thermal plasticity of ectothermic animals as regulators of resting membrane potential and modulators of action potential duration. They form functional channels as homo- and heterotetrameres allowing potassium current (IK1) with channel type and intracellular rectification factors (IRFs) dependent rectification properties. We have tested the role of Kir2.x channels by using electrophysiological and molecular biology techniques and by comparing functions of endogenous ion channels of fish (crucian carp, Carassius carassius, and rainbow trout, Oncorhynhus mykiss) and recombinant fish channel proteins expressed in mammalian cell lines. We have also used electrophysiological and molecular modeling to characterize the physiological importance and structure related functionality, respective. We have cloned 7 members of Kir2 family from fish including a novel Kir2 protein: ccKir2.5. The novel channel is the predominant IK1 forming channel in cold acclimated crucian carp and has the steepest inward rectification among Kir2 family. Furthermore, we have showed with quantitative RT-PCR and single channel patch-clamp methods the acclimation dependency of expression of Kir2.x coding genes and functional proteins in both fish species. Our results suggest that composition of IK1 mediating Kir2.x channels can be important for thermal plasticity of animals. However, the mechanical explanation of seasonal switch between different Kir2.x channels in cardiac myocytes remains to be elucidated.