McGregor will be looking to avenge his submission loss to Diaz in the second round of UFC 196.
UFC anti-doping partner USADA confirmed Lesnar failed the test conducted on 28 June.
Ronda Rousey will be guaranteed a title shot on comeback, according to UFC President Dana White.
UFC 200 generated $10.8m in ticket sales and searches for betting odds have soared 1000% in two years.
Ultimate Fighting Championship sold to talent agency WME-IMG for a reported $4bn.
Eddie Alvarez beat Rafael dos Anjos to win the lightweight title at UFC Fight Night 90.
Randy Orton and Brock Lesnar will square off at SummerSlam in August 2016.
New owners WME-IMG is best known as a talent and sports agency headed by Ari Emanuel and Patrick Whitesell.
Brock Lesnar gave no indication on whether he will fight again in the UFC.
Brock Lesnar returns to UFC for a one-off fight against Mark Hunt at the T-Mobile Arena on 9 July.
Miesha Tate defends her bantamweight title for the first time since beating Holly Holm at UFC 196.
Jones pulled from card and is under investigation by USADA after failing drug test.
Interim champion being investigated by USADA and pulled from UFC 200's main event.
The 28-year-old tested positive for a banned substance in an out-of-competition sample.
Conor McGregor and Floyd Mayweather bout is unlikely to take place.
Miesha Tate will defend her bantamweight championship title against Amanda Nunes at UFC 202.
Jon Jones set to resume his career following suspension in 2015.
Ronda Rousey unlikely to make a comeback at UFC 205.
Conor McGregor will take on Nate Diaz in a rematch of UFC 196 on 20 August in Las Vegas.
Brock Lesnar will headline UFC 200 as he takes on Mark Hunt in the co-main event on 9 July.
UFC Bantamweight champion Miesha Tate reveals her dream of beating Ronda Rousey.
Ambrose will be involved in Money In The Bank PPV, which is set to take place on 19 June.