<ul class="dropdown-menu scrollable-menu" role="menu">
          <li><a rel="nofollow" href="#">Action</a></li>
          <li><a rel="nofollow" href="#">Another action</a></li>
          <li><a rel="nofollow" href="#">Something else here</a></li>
          <li><a rel="nofollow" href="#">Action</a></li>
            ..
          <li><a rel="nofollow" href="#">Action</a></li>
          <li><a rel="nofollow" href="#">Another action</a></li>
       </ul>

.scrollable-menu {
    height: auto;
    max-height: 200px;
    overflow-x: hidden;
}