  We present a regularization procedure of period integrals of automorphic
forms on a group $G$ over an arbitrary reductive subgroup $G' \subset G$. As a
consequence we obtain an explicit $G'(\mathbb{A})$-invariant functional on the
space of automorphic forms on $G$ whose exponents avoid certain prescribed
hyperplanes. We also provide a necessary and sufficient condition for
convergence of period integrals of automorphic forms in terms of their
exponents.
