Adult male Wistar rats were continuously irradiated for 30 days on an experimental field from a 60Co source or radiation. Lipoprotein lipase activity was determined in their adipose tissue, heart and liver at intervals of 1, 3, 7, 14, 21 and 30 days from the beginning of irradiation and triacylglycerol, total cholesterol, phospholipid and non-esterified fatty acid concentrations were determined in their serum. Throughout the whole of the study, lipoprotein lipase activity was lower in the adipose tissue and higher in the heart of irradiated rats than in the controls. In the liver it was low 3 days from the onset of irradiation; at the other intervals it was variable and differed only non-significantly from the controls. Serum lipid concentrations were raised in irradiated rats--triacylglycerol from the 7th day, phospholipids from the 14th day and non-esterified fatty acids throughout the whole period of irradiation. In keeping with the high triacylglycerol values in the serum of irradiated rats, lipoprotein lipase activity in their adipose tissue was low.