His recent GQ cover has made it clear: Joseph Gordon-Levitt is kinda jacked. The 31-year-old former "Third Rock from the Sun" kid has grown up to be a bona fide action star, debuting bulging biceps in the much-anticipated Christopher Nolan flick "Dark Knight Rises," which opens in theaters Friday, July 20.

But Gordon-Levitt isn't the only male celebrity to have undergone a transformation, onscreen and off. Check out our gallery of male stars who have gone from lean to mean, including Christian Bale, Mickey Rourke, Zac Efron and more.

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  • Christian Bale

    Christian Bale should win an Academy Award alone for the number of times he's yo-yo'd for movie roles -- dropping, then regaining, huge amounts of weight and muscle for roles in "The Machinist," "Rescue Dawn," "The Fighter" and the "Batman" series.

  • Joseph Gordon-Levitt

    Yep, the lanky, little kid from "Third Rock From the Sun" has turned into a bona fide action star who shows off his new biceps in "Dark Knight Rises."

  • Joey Lawrence

    All we have to say about the "Blossom" star's jacked physique -- which he debuted as a <a href="http://www.huffingtonpost.com/2012/06/09/joey-lawrence-chippendales-blossom-star-las-vegas_n_1583518.html" target="_hplink">celebrity Chippendales dancer in June</a> -- is "WHOA!"

  • Mickey Rourke

    The comeback of the decade came when '90s star Mickey Rourke returned to the big screen in Darren Aronofsky's 2008 drama, "The Wrestler."

  • Ryan Gosling

    Hey guy, when did you become so buff?

  • Zac Efron

    Zac Efron bulked up when he underwent intense physical training for his role as a U.S. Marine in "The Lucky One." <a href="http://www.huffingtonpost.com/2012/04/13/zac-efron-lucky-one_n_1423678.html" target="_hplink">He told <em>Men's Health</em> in April</a>, "By the end of the movie I didn't recognize myself... You hear about guys like Christian Bale who dive into it and are really able to transform. I've always wondered if I had the willpower to actually do it. And I'll always have pride around the sense that I can."

  • Chris Evans

    Captain America's transformation from a scrawny kid to a pumped-up superhero wasn't a far stretch from Chris Evans' own evolution. The actor said his<a href="http://www.huffingtonpost.com/2011/07/19/chris-evans-talks-captain-america-workout_n_903919.html" target="_hplink"> daily, two-hour workouts were brutal</a>, but that eating to gain weight "sucked more": "You're just so bloated, but you just have to keep consuming protein, so that was tricky."