Prolonged anoxia-tolerance of facultative anaerobes is based on metabolic depression, and thus on controlled reduction of energy-utilizing processes. One proposed survival mechanism is the closing of ion channels to decrease energetic cost of ion pumping (8). To test this hypothesis, the involvement of L-type Ca²⁺ channels in seasonal anoxia-tolerance of the vertebrate heart was examined by determining the number of [methyl-³H]PN200-110 (a ligand of L-type Ca²⁺ channel alpha subunit) binding sites of the cardiac tissue and the density of Ca²⁺ current in ventricular myocytes of an anoxia-resistant fish species, the crucian carp. In their natural environment, the fish were exposed for over 3 months of hypoxia (O₂ <2.5 mg/L) followed by almost 8 weeks of anoxia which resulted in abrupt depletion of cardiac glycogen stores in late spring. Unexpectedly, however, the number of [methyl-³H]PN200-110 binding sites did not decline in hypoxia/anoxia as predicted by the channel arrest hypothesis, but remained constant for most of the year. However, in early summer, the number of [methyl-³H]PN200-110 binding sites doubled for a period of about two months which functionally appeared as a 74% larger Ca²⁺ current density. Thus, the anoxia tolerance of the carp heart cannot be based on down-regulation of Ca²⁺ channel units in myocytes, but is likely to depend on suppressed heart rate, i.e. regulation of the heart at the systemic level, and direct depressive effects of low temperature on Ca²⁺ current to achieve savings in cardiac work load and ion pumping. The summer peak in the number of functional Ca²⁺ channels indicates a short period of high cardiac activity possible associated with reproduction and active perfusion of tissues after the winter stresses.