  Using instanton calculus we check, in the weak coupling region, the
nonperturbative relation $$ <\Tr\phi^2>=i\pi\left(\cf-{a\over 2}
{\partial\cf\over\partial a}\right)$$ obtained for a N=2 globally
supersymmetric gauge theory. Our computations are performed for instantons of
winding number k, up to k=2 and turn out to agree with previous nonperturbative
results.
