The aim of the present study was to examine the effect of different kinds o
f physical exercise on plasma glutathione levels. Male Wistar rats were ran
domly divided into four groups: In walking group (W; n=6), rats were traine
d to walk 0.8 m/min for 45 min; slow running group (SR; n=6) were trained t
o run 4 m/min for 45 min; fast running group (FR; n=6) ran 8m/min for 60 mi
n and control rats (C; n=6) remained in their home cages. All animals were
sacrificed after exercise and the levels of reduced glutathione (GSH) in pl
asma samples determined by high performance liquid chromatography (HPLC) wi
th a fluorescent detector. Compared to controls, exercise did not change GS
H plasma levels of the W group. A tendency to decrease blood GSH was observ
ed in plasma samples of the SR group and in the FR group, physical exercise
resulted in a dramatic decrease in GSH plasma levels. These data suggest t
hat during light physical exercise there is a low production of reactive ox
ygen species (ROS) with a low request for antioxidant defence such as oxida
tion of GSH. The dramatic decrease observed in GSH levels in FR rats would
indicate the presence of oxidative stress able to modify blood antioxidant
profiles. Our results suggest that GSH plays a central antioxidant role in
blood during intensive physical exercise and that its modifications are clo
sely related to exercise intensity.