sentences:  After the Senate election of 1963 Zijlstra was elected as a Member of the Senate, taking office on 25 June 1963 serving as a frontbencher.Following the cabinet formation of 1963 Zijlstra per his own request asked not to be considered for a cabinet post in the new cabinet, the Cabinet De Quay was replaced by the Cabinet Marijnen on 24 July 1963.Zijlstra returned as a distinguished professor of Public economics at the Free University Amsterdam serving from 1 January 1964 until 1 May 1967.