It has been firmly established that the longevity of 20- to 60%-calorie-restricted rodents, with malnutrition (essential nutrients deficiency) being avoided, is increased when compared to ad libitum fed rodents. However, the effects on life span of severe dietary restriction (i. e. malnutrition), with limited weight loss, remained unknown. The purpose of this 4-year study was to investigate the effects on longevity of a severe form of dietary restriction, with limited and controlled weight loss. To this end, a group of male Long-Evans rats severely dietary restricted (SDR group), with a weight loss throughout the experiment<or=25% of their weight before the onset of the experiment at 9 weeks of age, was compared to a control group of rats 30- to 40%-calorie-restricted (C group). Our results show that a severe dietary restriction, excessive weight loss being prevented, paradoxically increased rat longevity by nearly 50%. The life span increase observed in our SDR rats is in accordance with some other studies investigating the effects on longevity of partial essential nutrients deficiencies (tryptophan, methionine, and fat, for example).