Several freshwater fishes of north-temperate latitudes exhibit marked seasonal changes in electrical activity as an outcome of temperature-dependent changes in the density of major ionic currents: delayed rectifiers (IKr,IKs) and inward rectifiers(IK1). In winter-acclimatized (WA) navaga cod (Eleginus navaga) marked overexpression of Kir and especially HERG channels leads respectively to increase in IK1 and IKr current density and resulting compensatory shortening of action potential (AP). Another current from Kir family, IKACh, which is activated exclusively due to the activation of cardiac muscarinic receptors, has never been shown to have seasonal variations. The present study was aimed to examine the potential seasonal differences of IKACh in navaga atrial myocytes and electrophysiological cholinergic effects associated with muscarinic stimulation. The study was performed using the whole-cell patch clamp technique and the standard sharp microelectrode technique of intracellular electrical activity registration. The fish were stunned by a blow to the head and decapitated. Then the multicellular preparation of atrium or the isolated atrial myocytes were obtained from excised navaga hearts. In atrial preparations from summer-acclimatized (SA) navaga 10-6M carbamylcholine chloride (CCh) produced slowing of spontaneous rhythm together with AP shortening and drastic reduction of AP amplitude down to the complete inexcitability. However, in preparations from WA navaga CCh slowed down the rhythm and reduced AP duration, but AP amplitude reduction was modest and never lead to inexcitability of atrial myocardium. This striking difference between SA and WA navaga could be well explained with respective seasonal changes in IKACh size. The density of IKACh induced by 10-5M CCh and measured at 0 mV was 2.8±0.5 pA/pF in SA navaga, but only 0.26±0.11 pA/pF in WA navaga. Thus, WA induces more than 10-fold decrease in IKACh density, which results in blunted cholinergic response of atrial myocardium in comparison with SA fish. To our knowledge IKACh is the only current in the fish heart, which decreases during the winter acclimatization.