After Independence, the first Constituent Assembly of Pakistan, elected in December 1945 in united India, was assigned the task of framing the Constitution of Pakistan. This Assembly passed the Objectives Resolution on 12th March, 1949, laying down principles which later became substantive part of the Constitution of Pakistan and still serves as its preamble. However, before it could accomplish the task of framing the constitution, the assembly was dissolved in October, 1954. Thereafter, the Governor General, convened the Second Constituent Assembly in May, 1955, which framed and passed the first Constitution of Pakistan on 29th February, 1956. That Constitution was promulgated on 23rd March 1956, which provided for a parliamentary form of Government with a unicameral legislature. However, from 14th August 1947 to 1st March 1956 the Government of India Act, 1935, was retained as the Constitution of Pakistan.
On October 7, 1958, Martial Law was promulgated and the Constitution abrogated. The Military Government appointed a Constitution Commission in February, 1960 which framed the 1962 Constitution. That Constitution provided for a Presidential form of Government with a unicameral legislature. The 1962 Constitution was abrogated on 25th March, 1969. The Civil Government, which came to power in December 1971 pursuant to 1970 elections, gave the nation an interim Constitution in the year 1972.
The 1970 Assembly framed the 1973 Constitution which was passed on 12th April and promulgated on 14th August 1973. The 1973 Constitution provides for a parliamentary form of Government with a bicameral legislature, comprising of the National Assembly and the Senate.
The membership of the Senate, which was originally 45, was raised to 63 in 1977 and to 87 in 1985. The membership of the Senate was again raised from 87 to 100 in 2002.