Sudden infant death is a problem of paediatric public health which remains an unpredictable phenomenon of unknown aetiology. Many authors have discussed the role of a neurovegetative aetiology, the role of the vagus nerve being suspected above all. A biological approach to vagal tone was attempted by assaying intragastric pepsin and serum pepsinogen and calculating the ratio of pepsin to acid in control infants, and by assaying serum pepsinogen levels in infants having suffered an apparently life-threatening event (ALTE) or who died of the sudden infant death syndrome (SIDS). This study showed that the age of 2 months appears to be a turning point in the development of digestive secretions, an age when the risk of SIDS is at its peak, and that the rates of the selected biological markers are significantly higher in victims of SIDS or ALTE than in controls. The predictive value of these biological markers is currently under evaluation.