  Let $(R,\frak m)$ be a commutative Noetherian local ring and let $M$ and $N$
be finitely generated $R$-modules of finite injective dimension and finite
Gorenstein injective dimension, respectively. In this paper we prove a
generalization of Ischebeck Formula, that is $\depth_RM+\sup\{i|
{0.1cm}\Ext_R^i(M,N)\neq 0\}=\depth R.$
