Plasma and lipoprotein cholesterol and triglycerides, and apolipoproteins AI and B, were determined in five common breeds of pet dogs, labrador, beagle, West Highland terrier, cairn terrier and dachshund, kept under carefully controlled conditions approximating as closely as possible to normal domestic living. Although the total levels of both cholesterol and triglycerides in plasma did not vary significantly between the breeds, there were significant interbreed differences in the lipoprotein fractions. Thus high density lipoproteins (HDL) which are the major carriers of cholesterol in all dogs, was higher in the smallest breed, the cairn, than in the largest, the labrador. In contrast, the low density lipoprotein (LDL) was lowest in the cairn and highest in the labrador. Variation in HDL cholesterol levels arose from both subfractions, HDL2 and HDL3. Thus metabolic studies involving canine lipids should include measurement of lipoprotein lipids and take account of interbreed variation.