Sixteen non-obese, non-smoking males, ages 20-30 yr, were assigned to one of two training groups, exercising on a cycle ergometer 3 d/wk for 18 wk: high-intensity (H; N = 7; 80-85% Vo2max, 25 min/session) or low-intensity (L; N = 9; 45% VO2max, 50/min/session). Data were obtained at 3-wk intervals for Vo2max, body weight, percent body fat, and 12-h fasting blood levels of cholesterol (CHOL), triglycerides (TG), high-density lipoprotein cholesterol (HDL-C), and low-density lipoprotein cholesterol (LDL-C). The average post-training increase in VO2max for group H (0.56 l X min-1, 8.5 ml X min-1 X kg-1) was not significantly (P greater than 0.05) greater than for group L (0.45 l X min-1, 6.5 ml X min-1 X kg-1). Significant reductions in percent body fat occurred in both groups, amounting to an average fat loss of approximately 1.35 kg. No statistically significant changes in CHOL, TG, HDL-C, LDL-C, CHOL/HDL-C, or HDL-C/LDL-C occurred in either group. However, changes in HDL-C after 18 wk of training were inversely correlated (r = -0.57, P less than 0.05) with pre-training levels. We conclude that 1) the minimum exercise training-intensity threshold for improving aerobic capacity is at least 45% Vo2max; 2) 18 wk of high- or low-intensity exercise training is ineffective in significantly altering CHOL, TG, HDL-C, LDL-C, CHOL/HDL-C, and HDL-C/LDL-C in young male subjects with low blood lipid levels, and 3) exercise training-induced changes in HDL-C are dependent upon initial pre-training levels.