Evidence that changes in feeding style alter the membrane fatty acid composition of ruminant tissue is presented here by comparing zoo giraffe with the same species from their natural habitat. The membrane changes seen are similar to those used experimentally to make animals susceptible to basic brain protein and encephalomalacia. Similar membrane responses have been noted in cattle. Use of animal protein and increased nitrogen in cattle feeds would lead to a relative deficiency of essential fatty acids in the cell membranes and hence reduced membrane stability. By analogy with crazy chick disease (nutritional encephalomalacia) and experimental encephalomyelitis in rats, the possibility that the changes in animals feeds would have depleted cattle tissue membranes and made them susceptible to BSE is discussed. The assumption being made is that the principle of a requirement of essential fatty acids for neural integrity and immune system function would apply to cattle as well as to other species.