The effects of 10 wk of exercise training at low (40% VO2max) or high (75% VO2max) intensity on serum lipids and lipoproteins were compared in prepubescent boys and adult men. The final sample size consisted of: 8 boys (mean +/- SE age = 8.5 +/- 1.96 yr) and 8 men (36.6 +/- 3.18 yr) in low; 12 boys (8.0 +/- 1.40 yr) and 12 men (36.6 +/- 4.09 yr) in high; and 10 boys (9.0 +/- 2.08 yr) and 10 men (36.7 +/- 4.82 yr) in control. Training involved walking/jogging/running 3 d X wk-1 at a distance which progressed from 2.4 km X d-1 in the first week to 4.8 km X d-1 from the fifth week. Fasting blood samples, collected on 2 d during both pre- and post-training, were assayed for triglycerides, total cholesterol (CHOL), and high density lipoprotein cholesterol (HDL-C). Maximum aerobic power (VO2max) was determined from a treadmill test. Additionally, dietary intake was assessed from a 3-d dietary record and body composition from the sum of 6 skinfolds. The only statistically significant (P less than 0.05) changes occurred in HDL-C and CHOL for the high groups. HDL-C decreased following training. CHOL was lower for high than the other groups for the first day post-training only. There were no differences in the changes in HDL-C/CHOL ratio among the groups. VO2max only increased in the high groups. Dietary intake and body weight did not change. Further statistical adjustment in lipids for changes in sum of 6 skinfolds did not alter the results.(ABSTRACT TRUNCATED AT 250 WORDS)