  Let $V$ be a $2n$-dimensional vector space over a field $F$ and $\Omega$ be a
non-degenerate symplectic form on $V$. Denote by ${\mathfrak H}_{k}(\Omega)$
the set of all $2k$-dimensional subspaces $U\subset V$ such that the
restriction $\Omega|_{U}$ is non-degenerate. Our main result (Theorem 1) says
that if $n\ne 2k$ and $\max(k,n-k)\ge 5$ then any bijective transformation of
${\mathfrak H}_{k}(\Omega)$ preserving the class of base subsets is induced by
a semi-simplectic automorphism of $V$. For the case when $n\ne 2k$ this fails,
but we have a weak version of this result (Theorem 2). If the characteristic of
$F$ is not equal to 2 then there is a one-to-one correspondence between
elements of ${\mathfrak H}_{k}(\Omega)$ and symplectic $(2k,2n-2k)$-involutions
and Theorem 1 can be formulated as follows: for the case when $n\ne 2k$ and
$\max(k,n-k)\ge 5$ any commutativity preserving bijective transformation of the
set of symplectic $(2k,2n-2k)$-involutions can be extended to an automorphism
of the symplectic group.
