The WHHL-rabbit (Watanabe-heritable hyperlipidemic rabbit) is a strain of rabbit with a consistently inherited hyperlipidemic trait produced by inbreeding from a mutant discovered in 1973. (1) The alleles for this trait have been inherited unfailingly to yield a total of 154 WHHL-rabbits to date. (2) These rabbits showed abnormally increased serum levels of cholesterol (S-Ch) and triglyceride (S-TG) at various ages, increased approximately 8- to 14-fold in comparison with control levels in normal Japanese white rabbits. The serum lipoproteins displayed an electrophoretic pattern characterized by a broad beta-lipoprotein band a markedly diminished alpha-lipoprotein band. (3) Necropsy examination performed on 75 WHHL-rabbits showed spontaneous development of aortic atherosclerosis in all cases over 5 months of age (63/63) and xanthoma of digital joints in 60% of cases from the ages of about 16 months.