Kim Kardashian has just served Kris Humphries' ex-girlfriend, Myla Sinanaj, with a subpoena in the divorce case ... TMZ has learned.

As TMZ first reported, Myla was in a boyfriend/girlfriend relationship with Kris since mid-January -- something Kris was desperately trying to hide ... to the point his people lied about the relationship.

As for why Kris wanted to cover it up ... sources tell us Kris and his lawyer were worried it would have an impact on the divorce case. We know Kris demanded $7 million from Kim, claiming she devastated him by filing divorce papers. It's hard to say he was devastated when he moved on two months after divorce docs were filed.

Sources say Kim's lawyer wants to know if Kris said anything about his marriage which might relate to his claim Kim defrauded him -- in other words, did he blow his cover during pillow talk. We're told Kim's lawyer also wants details about hundreds of texts and emails exchanged between Myla and Kris.

Myla's lawyer, Joe Tacopina, accepted service on behalf of Myla.

Tacopina tells TMZ his client will comply with the subpoena.

Mel Gibson blew off some serious steam Saturday night, partying until 4 AM at Maria Menounos' home.

Maria threw a bash for her 34th birthday. A wardrobe friend of Maria's brought Mel along and he quickly made himself at home in her Encino pad, grinding away without a care in the world.

As for the question, "WHO THE F**K WANTS TO EAT?," Mel certainly did, because girls in tiny grass skirts were serving hors d'oeuvres all night long.

BTW, the party had a Hawaiian theme, so Mel was greeted at the door with a lei -- we're guessing she wasn't Russian.

Beach season is officially here -- and we have the photos to prove it!

Stars like Miley Cyrus, Joe Manganiello, Chris Hemsworth, Fergie and the cast of "Jersey Shore" have already stripped down to their swimsuits.

Before this little lady grew up to be one of Hollywood's most sultry singers, she was just another girl growing up in New York! Can you guess who this mother of two is?!

Tim McGraw is 45 and fine!

The country superstar proved that wife Faith Hill is a lucky lady this weekend -- when he posted a shirtless photo of himself on Twitter.

Which other stars are still lookin' good after 40?

Can you guess which Hollywood hotties are the proud owners of these taught tums? Check out all the man muscles ... if you can stomach them!

Alright stop ... collaborate and focus! We've got a set of differences for you to check out while the DJ revolves it!

**HINT -- There are three differences in the above photographs!**

Rihanna is running out of real estate on her body -- as she got yet another tattoo!