Kylie Jenner & Stassie Karanikolaou might have unimpressed expressions on their faces, but they're actually bursting with excitement ... or, at least, nearly busting out of their tops!

Jenner shared several pics with her longtime pal Saturday ... showing the two sitting side by side in a car with tight white tank tops, leaving little to the imagination!

Check out the pics ... the two dark-haired vixens are pulling out all their modeling experience for this hot impromptu photo shoot -- pouty lips and serious expressions galore.

Stassie does break into a grin at one point, showing off her sparkling pearly whites ... though Kylie's intent on keeping her composure in the photo.

Kylie captured the photos, "🤍♾️" ... to which Stassie replied in the comments, "our song 🥹🥹🫶🏼🫶🏼🫶🏼." We don't fully get the meaning -- but, that's just what happens when you hear two besties talking to one another!

Kylie's all about barely-there tops these days, it seems ... remember, we shared video yesterday of Jenner's boobs stealing the show during a makeup tutorial -- though she went on the other end of the color spectrum for that busty look.

Seems Stassie down to show off alongside her pal ... and, we know their fans ain't complaining one bit!