  In this work we investigate the following isoperimetric problem: to find the
regions of prescribed volume with minimal boundary area between two parallel
horospheres in hyperbolic 3-space (the area of the part of the boundary
contained in the horospheres is not included). We reduce the problem to the
study of rotationally invariant regions and obtain the possible isoperimetric
solutions by studying the behaviour of the profile curves of the rotational
surfaces with constant mean curvature in the hyperbolic space.
