Diet restriction of rodents during adult life is known to cause an increased life span. It has been hypothesised that this increase may be related to effects on the anti-oxidant defence systems. However, it has been suggested that undernutrition during the gestation and pre-weaning may reduce their life span as it is known to have other deleterious effects on a rodent's growth and development. We have now examined the activity levels of some anti-oxidant defence system enzymes and other markers of oxidative stress in mice that have been undernourished from conception until 21 postnatal days of age, followed in some cases by a period of nutritional rehabilitation until 61 days of age. We found that such undernutrition exerted only minimal effects on oxidative stress markers under investigation (ROS enzyme activities, GSH levels, and lipid peroxidation). Only GSH levels were significantly affected by pre-weaning undernutrition. In conclusion, pre-weaning undernutrition may regulate anti-oxidant enzymes at the transcriptional level differently from that at the post-transcriptional, translational, or post-translational levels. The possible effects that these changes at the cellular level, may have on the longevity of the animals remain of great interest and importance.