The European badger has previously been shown to exhibit yearly cycles of locomotor activity, endocrine secretions, and body weight, as well as seasonal variations in plasma cholesterol. Over a period of 2 years, we have followed the plasma levels of free and esterified cholesterol, triglycerides and phospholipids, and of plasma lipoproteins (by means of polyacrylamide gel electrophoresis, agarose column chromatography and preparative and analytical ultracentrifugation). Some preliminary observations on the qualitative characteristics of the plasma apoproteins, obtained by application of electrophoretic techniques, are also described. Our results provide evidence for considerable synchronous and spontaneous variations of each of the plasma lipid components studied, all of them reaching a maximum in late autumn/early winter, then decreasing to a minimum in early spring. In some animals, the amplitude of observed variations was as large as 650% for total cholesterol, 420% for phospholipids and 180% for triglycerides. While the plasma concentration of very low density lipoproteins (d < 1.006 g/ml) remained at low or moderate levels, major changes in the lipoprotein spectrum occurred in the low density (1.006--1.063 g/ml) and high density (1.063--1.21 g/ml) lipoproteins, these two classes exhibiting marked heterogeneity. This led to an autumn/winter prominence of the 1,006--1.063 g/ml components and of those in the lower part of the high density range, with an enrichment in cholesterol in lipoproteins in the low density region. These phenomena occur simultaneously and/or immediately after the annual minimum of plasma thyroxine concentration in the species considered. In contrast, early spring patterns displayed more classical features with higher density lipoproteins predominating. Our findings thus suggest that the badger may provide a useful model for future experiments regarding the hormonal regulation of plasma lipid transport as well as the metabolism and physiopathological implications of some cholesterol-rich lipoproteins.