  In analogy with the study of representations of $GL_{2n}(F)$ distinguished by
$Sp_{2n}(F)$, where $F$ is a local field, in this paper we study
representations of $U_{2n}(F)$ distinguished by $Sp_{2n}(F)$. (Only quasi-split
unitary groups are considered in this paper since they are the only ones which
contain $Sp_{2n}(F)$.) We prove that there are no cuspidal representations of
$U_{2n}(F)$ distinguished by $Sp_{2n}(F)$ for $F$ a non-archimedean local
field. We also prove the corresponding global theorem that there are no
cuspidal representations of $U_{2n}({\mathbb A}_k)$ with nonzero period
integral on $Sp_{2n}(k) \backslash Sp_{2n}({\mathbb A}_k)$ for $k$ any number
field or a function field. We completely classify representations of
quasi-split unitary group in four variables over local and global fields with
nontrivial symplectic periods using methods of theta correspondence. We propose
a conjectural answer for the classification of all representations of a
quasi-split unitary group distinguished by $Sp_{2n}(F)$.
