  For a symmetric pair $(G,H)$ of reductive groups we construct a family of
intertwining operators between spherical principal series representations of
$G$ and $H$ that are induced from parabolic subgroups satisfying certain
compatibility conditions. The operators are given explicitly in terms of their
integral kernels and we prove convergence of the integrals for an open set of
parameters and meromorphic continuation. We further discuss uniqueness of
intertwining operators, and for the rank one cases $$
(G,H)=(SU(1,n;\mathbb{F}),S(U(1,m;\mathbb{F})\times U(n-m;\mathbb{F}))), \qquad
\mathbb{F}=\mathbb{R},\mathbb{C},\mathbb{H},\mathbb{O}, $$ and for the pair $$
(G,H)=(GL(4n,\mathbb{R}),GL(2n,\mathbb{C})) $$ we show that for a certain
choice of maximal parabolic subgroups our operators generically span the space
of intertwiners.
