Kourtney Kardashian is getting serious about her fitness.

The reality TV star hit the gym for the first time in a while on Tuesday, revealing she's ready to whip her body back into tip-top shape.

"Day one," she posted with the photo above, showing a makeup-free Kourt by the free-weights.

She later revealed that her inspiration for breaking a sweat was seeing an old pic of herself in a bikini, which she shared on her Instagram page.

"This is why I started working out today," she wrote along with the sexy snap.

Kourtney isn't the only one in her family feeling the burn these days. Both Kim Kardashan and Kendall Jenner have also shared some revealing pics from the gym lately.

Check out the gallery above to see what other celebs look like without makeup on -- some of 'em may surprise you.

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