Brock Lesnar is still under contract to UFC, according to MMAFighting.com. It's been reported, and stated by Dana White, that Lesnar's future in the Octagon depends on Vince McMahon and WWE as Lesnar is still under contract to WWE but MMAFighting.com has confirmed, via UFC officials, that Lesnar is also still signed to a UFC deal.
UFC rules state that a fighter must inform the promotion 4 months in advance if he or she is coming out of retirement, so they can be placed into the USADA testing pool. In a controversial move, this rule was waived by UFC for Lesnar's UFC 200 return but he was tested 6 times in the 4 weeks leading up to the event, but was not required to do the 4 months of testing. Lesnar received the exemption because he didn't retire while USADA was overseeing UFC's anti-doping policy, and because a long time had passed since Lesnar left the promotion. Lesnar was essentially treated as if he were a new athlete that just signed with the company.
There's still no word yet on Lesnar's UFC future but there have been rumors this week that he is eyeing another fight later this year or early 2017. Lesnar returns to the ring for WWE on August 21st at SummerSlam, against Randy Orton.