Twelve randomly-selected UFC 81 fighters have passed their mandatory drug tests according to MMA Junkie. The fighters were tested by the Nevada State Athletic Commission for performance enhancing substances and drugs of abuse.
Tested Fighters

Frank Mir
Brock Lesnar
Antonio Rodrigo Nogueira
Tim Sylvia
Ricardo Almeida
Tim Boetsch
Marvin Eastman
Rob Emerson
Tyson Griffin
Jeremy Horn
Chris Lytle
Nate Marquardt

Tim Boetsch has again stepped up on short notice and will face Matt Hamill  at UFC Fight Night 13 on April 2nd. MMA Weekly confirmed the replacement with a source close to the fight.
Additionally, UFC.com has added a bout between “The Ultimate Fighter 6″ cast members Roman Mitichyan and George Sotiropoulos to the preliminary card [...]

Antonio Rodrigo Nogueira def. Tim Sylvia
Nogueira — $200,000 ($100,000 to show; $100,000 to win)
Sylvia — $100,000
Frank Mir def. Brock Lesnar
Mir — $80,000 ($40,000 to show; $40,000 to win)
Lesnar — $250,000
Nate Marquardt def. Jeremy Horn
Marquardt — $52,000 ($26,000 to show; $26,000 to win)
Horn — $25,000
Ricardo Almeida def. Rob Yundt
Almeida — $40,000 ($20,000 to show; $20,000 to [...]

Antonio Rodrigo Nogueira def. Tim Sylvia via submission – Round 3, 1:28 (Nogueira becomes the Interim UFC Heavyweight Champion)
Frank Mir def. Brock Lesnar via submission – Round 1, 1:30
Nate Marquardt def. Jeremy Horn via submission – Round 2, 1:37
Ricardo Almeida def. Rob Yundt via submission – Round 1, 1:08
Tyson Griffin def. Gleison Tibau via unanimous [...]

UFC 81 “Breaking Point” took place Saturday, February 2nd at The Mandalay Bay Events Center in Las Vegas, Nevada.
The Super Bowl Eve event was headlined by a pair of heavyweight showdowns. In the main event, former UFC champ Tim Sylvia faced former PRIDE title holder Antonio Rodrigo Nogueira for the Interim UFC Heavyweight Championship. The [...]

265 lb.: Antonio Rodrigo Nogueira (237) vs. Tim Sylvia (261)
Interim UFC Heavyweight Championship Bout
265 lb.: Frank Mir (255) vs. Brock Lesnar (265)

185 lb.: Jeremy Horn (185) vs. Nate Marquardt (185)

185 lb.: Rob Yundt (185) vs. Ricardo Almeida (185)

155 lb.: Gleison Tibau (156) vs. Tyson Griffin (155)

170 lb.: Kyle Bradley (170) vs. Chris Lytle (170)

205 lb.: [...]

On Saturday night at UFC 81 a new Heavyweight champion will be crowned and the birth or re-birth of a career will happen for one particular fighter. Brock Lesnar will be looking to start a new career as a mixed martial arts fighter as he makes the crossover from entertainment-style pro wrestling to the fight [...]

From UFC.com’s Thomas Gerbasi:
February 2nd’s UFC 81 card in Las Vegas is back up to a full slate of nine fights after Pennsylvania light heavyweight prospect Tim Boetsch agreed to step into the Octagon to face David Heath. Boetsch (6-1 in pro MMA) replaces Tomasz Drwal, who was forced to withdraw from the Heath bout [...]