  Let F be a finite extension of Qp and G be GL(2,F). When V is the tensor
product of three admissible, irreducible, finite dimensional representations of
G, the space of G-invariant linear forms has dimension at most one. When a non
zero linear form exists, one wants to find an element of V which is not in its
kernel: this is a test vector. Gross and Prasad found explicit test vectors
when the three representations are unramified principal series, and when they
are all unramified twists of the Steinberg representation. In this paper we
decribe explicit test vectors when two of the representations are principal
series.
