Diepenhorst returned as a distinguished professor of Constitutional law, Administrative law and Governmental studies at the Free University Amsterdam serving from 23 February 1967 until 28 September 1984 and also as a distinguished professor of Parliamentary History from 1 September 1976 until 28 September 1984. He also served again as Rector Magnificus of the Free University Amsterdam from 1 September 1972 until 1 September 1976. In January 1971 Diepenhorst announced that he wouldn't stand for the election of 1971 but wanted tot return to the Senate. After the Senate election of 1971 Diepenhorst returned to the Senate, he resigned as a Member of the House of Representatives the day he was installed as a Member of the Senate, serving from 11 May 1971 until 17 September 1974 and again from 29 October 1974 until 10 June 1981 serving as a frontbencher chairing several parliamentary committees. Diepenhorst also became active in the public sector occupying numerous seats as a nonprofit director on several boards of directors and supervisory boards (Royal Academy of Arts and Sciences, Netherlands Bible Society, Bartiméus Foundation and the Utrecht University) and served on several state commissions]] and councils on behalf of the government (Education Council, Probation Agency and the Council for Culture).[2][3]