Naruto Shippūden: Season 2, Episode 40

2 - 40: The Nine-Tails Unleashed!!

Immediately after Yamato signals Naruto and the others out to the bridge where Orochimaru and Kabuto are waiting, Naruto's chakra begins to boil with the chakra of the Nine-Tailed Fox. Slowly, Naruto grows more tails as Orochimaru taunts him and makes him even more angry. Meanwhile, Yamato remembers Jiraiya speaking of how many tails he has seen Naruto grow. Jiraiya explains more about what happened and more about how Naruto acts when he's in his Nine-Tails form.