1955–1974: The county of Midlothian, including all the burghs situated therein, except the county of the city of Edinburgh and the burgh of Musselburgh.
1974–1983: As above.
1983–1997: Midlothian District.
1997–2005: The Midlothian District electoral wards of Bonnyrigg/Newtongrange, Dalkeith, Loanhead, and Mayfield/Gorebridge.
The 1997 reforms moved Penicuik into Tweeddale, Ettrick and Lauderdale, with the rest of the council area remaining in the seat;[2] it returned in 2005, when the constituency was expanded again to cover the entire Midlothian Council area.[3]