List of Vice-Presidents Who Served as "Acting" President Under the 25th Amendment Reagan - G.W. Bush
"Whenever the President transmits to the President pro tempore of the Senate and the Speaker of the House of Representatives his written declaration that he is unable to discharge the powers and duties of his office, and until he transmits to them a written declaration to the contrary, such powers and duties shall be discharged by the Vice President as Acting President." -- Constitution of the United States, Amendment 25, Section 3
Citation: John Woolley and Gerhard Peters,The American Presidency Project [online]. Santa Barbara, CA: University of California (hosted), Gerhard Peters (database). Available from World Wide Web: http://www.presidency.ucsb.edu/temporary_presidents.php.