According to this report Brock Lesnar has officially notified the UFC he has retired from MMA. If this is true it is not surprising, really. Lesnar is 39 and would not be eligible to enter the USADA testing pool again until later this year after being suspended following a failed drug test before and on the night of UFC 200. Regardless, Lesnar will be performing at WrestleMania 33 on April 2 in Orlando this year for WWE. He will not be hurting for money.
Brock nor the UFC as of this posting has made it official.