Abstract
A macromolecular “antifreeze” is present in the blood serum of fishes that inhabit subzero waters in winter. During the summer the “antifreeze” is not present. The effect of temperature and photoperiod on this circ‐annual cycle of “antifreeze” production was investigated. Fishes collected from subzero waters were acclimated to various temperature and photoperiod regimes. Only the combination of a long photoperiod and warm temperature resulted in the disappearance of “antifreeze” from the serum, thus providing for an efficient failsafe system. The time necessary for the complete loss of the macromolecular “antifreeze” ranged from 3 to 5 weeks. When summer fish were acclimated to various combinations of temperature and photoperiod, low temperatures resulted in the production of “antifreeze” regardless of the photo‐period. Serum Na and Cl^−^ levels were significantly higher in cold acclimated than in warm acclimated fish. In general, fishes collected from more northerly latitudes had lower serum freezing points than the same species from warmer, more southerly waters. There seemed to be a genetic difference between populations of Anoplarchus purpurescens from Amchitka, Alaska and San Simeon, California since the California fish were unable to produce “antifreeze” when acclimated to cold temperatures.