All organisms respond to environmental challenge by adaptive responses, although, in many cases, the underlying molecular mechanisms are not understood. In the case of membranes, the physical structure of membrane phospholipids is conserved in the face of cold, rigidifying conditions by the elevated proportions of unsaturated fatty acids. We have observed a clear positional specificity in this substitution and head group preferences in carp liver membranes. We have also demonstrated changes in the activity of lipid desaturases that mediate the unsaturation response, caused by both transcriptional and post-translational mechanisms. Another hepatic isoform has recently been discovered with sensitivity, not to cooling, but to dietary variations. Finally, we are testing the importance of desaturase inductions in the inducible cold tolerance of the whole animal.