ObjectiveTo determine whether plasma glutamine levels can be used as an indicator of exercise‐induced stress, and to consider the possible effects of low plasma glutamine concentrations on the immune system.MethodsWe used two exercise regimens: in Trial 1 seven male subjects were randomly stressed on a treadmill at 0, 30%, 60%, 90% and 120% of their maximal oxygen uptake $(Vo2max); in Trial 2 five highly trained male subjects underwent intensive interval training sessions twice daily for ten days, followed by a six‐day recovery period.ResultsPlasma glutamine concentrations decreased significantly from an average of 1244+ 121 μmol/L to 702 ± 101 μmo1/L after acute exercise at 90% $VDo2max (P<0.05) and to 560 ± 79 μmol/L at 120% $VDo2max (P<0.001). Four of the five subjects showed reduced plasma glutamine concentrations by Day 6 of the overload training trial, with all subjects displaying significantly lower glutamine levels by Day 11. However, glutamine levels showed a variable rate of recovery over the six‐day recovery period, with two subjects' levels remaining low by Day 16.ConclusionsReduced plasma glutamine concentrations may provide a good indication of severe exercise stress.