Membrane response to the various temperatures as one of the external factors was investigated in the lenses of the poikilothermal animal and the homothermal animal. The rainbow trout lens was used as the poikilothermal material and the rat lens as the homothermal material. The rainbow trout lens maintained in vitro its transparency without the changes of cation balance at 0 degrees C - 25 degrees C, while cold cataract developed in the rat lens under the same conditions. Na, K-ATPase activity was detected at 0 degrees C in rainbow trout lens but it was not detected in the rat lens. Lactic acid in the rainbow trout lens was produced for 30 successive days at 0 degrees C, while that in the rat lens was not produced. The cataract developed at 37 degrees C in rainbow trout lens, which we called "warm cataract." Warm cataract developed not only when the lens was incubated in vitro but also when rainbow trout was kept in fresh water at 37 degrees C. Significant differences were detected in components of membrane lipids in the rainbow trout lenses compared to bovine lenses as the mammalian lens. The cholesterol/phospholipid ratio in the trout lens membrane was lower than that in bovine lens. This suggests that a poikilothermal animal lens can maintain the membrane fluidity at low temperatures. These results might suggest that the membrane characteristics in the rainbow trout lens play a role to maintain its transparency at low temperatures.