Lowe last played Sam Seaborn in the show's fourth season and Parker's Amy Gardner was last seen in the 2003-2004 season.
Cole will revive his role as Vice President Bob Russell and Matheson will be back as former Vice President John Hoynes. Gish will reprise her role as President Bartlet's eldest daughter, Elizabeth, and Marlee Matlin will be back in the role of political consultant Joey Lucas.
"CSI: Miami" star Procter will revive her role as associate White House counsel Ainsley Hayes, Smith will return as National Security Adviser Nancy McNally and Timothy Busfield again portray journalist Danny Concanon.
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