On the rumor of Brock Lesnar possibly doing a UFC fight, word is that his deal with WWE is exclusive to them and doesn’t expire until WrestleMania 31 in 2015.
People in the know can’t believe that Vince McMahon would allow Lesnar to sign a deal with WWE that would allow him to do a UFC fight at the same time.
There you have it, as of right now Lesnar is to stay with WWE.
On a side note, Lesnar has not fought since 2011 and is 36. The chance of him fighting is slim to none. If he did go back to the UFC, he would have been out of the sports since 2011 and would be 38 years hold.