This study examined the effects of 6 months of resistance exercise (RX) on basal and post-aerobic exercise lipid peroxidation (LIPOX). Men and women [n = 62, mean (SD) age 68.4 (6) years] were divided randomly into either a control (n = 16, CON), low-intensity training [LEX n = 24; 50% one-repetition maximum (1RM), 13 repetitions/exercise], or high-intensity training (HEX n = 22, 80% 1RM, 8 repetitions/exercise) group. Pre- and post-training, subjects performed a graded aerobic exercise test (GXT). Blood samples were collected prior to and 10 min following each GXT. Subjects trained 3 times per week for 6 months using 12 RX machines. LIPOX was determined by measuring levels of thiobarbituric reactive acid substances (TBARS) and lipid hydroperoxides (PEROX). RX had no effect on resting LIPOX. Post-training, post-GXT TBARS were lower in the LEX and HEX groups by 14% and 18%, respectively, compared to CON (P < 0.05). Post-GXT PEROX levels were lower (P < 0.05) in LEX and HEX compared to CON [CON 3.51 (0.56) nmol/ml, LEX 2.89 (0.80) nmol/ml, HEX 2.99 (0.63) nmol/ml]. Serum total and non-protein (glutathione) thiols were higher in the LEX and HEX groups following training compared to CON (P < 0.05). These data suggest that RX can (1) reduce serum LIPOX, (2) provide protection against oxidizing agents in vitro, and (3) provide a "cross-protection" against the oxidative stress generated by aerobic exercise, perhaps mediated by improvements in the thiol portion of the antioxidant defense.