Treatment of large blood vessels with ionizing irradiation leads to the development of atheromatosis when the serum lipid levels are sufficiently high. In order to answer the question whether a disturbed lipid clearance in the arterial wall plays a role in the accumulation of the lipids it was of interest to examine the effect of X-irradiation on the lipase activity. Triglycerol lipase was tested in a homogenate from rabbit carotid arteries, glycerol-tri-oelate was used as a substrate and the hydrolysis assayed at pH 6.4 at 37 degrees C. Under the conditions used, an hydrolytic activity could be measured of 7.5 nmoles . mg prot-1 . hr-1. The lipase activity was also tested in the carotid arteries obtained from rabbits which had been locally irradiated with 20 Gy of X-rays. No clear radiation effect on the lipase activity was found 4, 8, 20, 24 and 72 hours after irradiation. The accumulation of lipids is thought to be caused by a higher influx of lipids from the serum as a result of endothelial damage by X-irradiation rather than a defect in the clearance capacity of the arterial wall.