  We consider the manifold $Fl_n(\mathbb{H})=Sp(n)/Sp(1)^n$ of all complete
flags in $\mathbb{H}^n$, where $\mathbb{H}$ is the skew-field of quaternions.
We study its equivariant $K$-theory rings with respect to the action of two
groups: $Sp(1)^n$ and a certain canonical subgroup $T:=(S^1)^n\subset Sp(1)^n$
(a maximal torus). For the first group action we obtain a
Goresky-Kottwitz-MacPherson type description. For the second one, we describe
the ring $K_T(Fl_n(\mathbb{H}))$ as a subring of $K_T(Sp(n)/T)$. This ring is
well known, since $Sp(n)/T$ is a complex flag variety.
