Longitudinal uniaxial stress along ⟨100⟩ has been applied in AlAs‐GaAs‐AlAs resonant tunneling heterostructures grown on (100) substrates to study the current‐voltage characteristics as a function of stress. We find that the nonresonant current is due to Fowler–Nordheim tunneling currents by both longitudinal and transverseXvalleys (X1andXt). This current decreases with low stress (<1 kbar), increases with intermediate stress and decreases with high stress (>9 kbar) again. This is due to the increase of theXtbarrier at low stress, the decrease of theX1barrier at intermediate stress, and the formation and the increase ofX1potential well depth at high stress. We also find a large unexplained monotonic decrease of the resonant current through the &Ggr; valley.