We calculate the scale dependence of nonsinglet nucleon structure, fun
ctions. Due to anomalous axial symmetry breaking a large flavour asymm
etry of the quark-antiquark sea is generated nonperturbatively. This p
roduces a strong scale dependence of the nonsinglet structure function
in an intermediate range of Q2 . Evolving nonperturbatively a pure va
lence distribution from an infrared scale we can thus compute F2p - F2
n as measured by the NMC, and give detailed predictions for its Q2 dep
endence at fixed x. We also compare our results with Drell-Yan data.