Tsunade begins to administer a shot of male hormones, once every few days, and then after a couple of weeks, gives him the shots to take home and administer himself. At first, he doesn't feel much different, but gradually the differences begin to appear. Sasuke notices first.
"You've got a hair."
"I've got lots of hairs." He shoots back, shoving his head in his friend's direction, pulling at the mass of blonde sprouting from his scalp.
"No, idiot, I meant—" He leans forward and tentatively pulls at Naruto's chin. He feels a twinge. "—you have a hair. On your face."
The grin that spreads across Naruto's face is ridiculously wide, as he runs to the mirror and examines the tiny blonde hair sprouting from his chin.
"A hair!" he smiles in disbelief, and Sasuke rolls his eyes but returns the smile.