Upper (Tf) and lower (Ts) temperature limits of order-disorder transitions in blood cell lipids of hedgehogs, Erinaceus europaeus, were determined over an annual cycle. There was a significant decrease in the temperature of both Tf and Ts from values of 19 and 6 degrees C, respectively, for summer animals to values of 14 and -2.0 degrees C for winter animals. Plasma thyroxine levels decreased from a summer mean of 16.0 nmol/liter to a mean of 2.3 nmol/liter in winter. Basal oxygen consumption also decreased from the summer mean 0.45 ml/g body wt/h to a mean 0.39 ml/g body wt/h in winter. In winter a group of hedgehogs kept indoors at room temperature was compared with a group kept outdoors exposed to natural winter conditions, and there was no significant difference between them in the above parameters. We conclude that the winter membrane lipid and metabolic changes are not a response to low temperature per se but part of a circannual homeostatic adjustment at least partly regulated by thyroid hormone.