Exposure of male rats to 0(air), 1, 1.75, and 3 ppm ozone (03) 5 hr/day for a total of 10 days resulted in a positive linear relationship between ozone concentration and the concentrations of serum total lipoprotein free cholesterol (FCh) and high-density lipoprotein total cholesterol (HDL-Ch). The latter response was reflected in both its free (HDL-FCh) and esterified (HDL-ChE) components. On the other hand, serum triglycerides (TG) showed a marked decreasing linear trend with increasing ozone concentration. As judged by decreased body weights with no accompanying differences in feed consumption, apparent metabolic rate increased as ozone concentration increased. In another experiment, male rats were exposed 5 hr/day to either air or 1 ppm 03 for a total of 15 days. Groups of animals from each exposure were sampled at times ranging from immediately after to 44 hr postexposure. In agreement with the concentration response study, effects of 03 included increases in serum total cholesterol (Ch), HDL-Ch and HDL-FCh, and a decrease in TG. In addition, the degree of effects appeared to be maintained over the 44-hr period and to be greater than that observed at 1 ppm 03 in the concentration-response study.