L-Citrulline is an amino acid that works to raise both arginine and ornithine plasma levels more efficiently and for longer periods of time than just l-arginine supplementation alone. It has quickly become the leader for nitric oxide increasing supplementation.
As an athletic performance supplement, l-citrulline has been shown to provide the following benefits when dosed properly (4-8g per serving).
Increased growth hormone (study showed 66.8% increase post workout) 
L-Citrulline is one of the most promising pre-workout supplement ingredients for the purpose of building muscle.
Derived from watermelon, l-citrulline is considered natural.
Serving Size: 1-2 Scoop (3-6g) Servings Per Container: 100-200
Directions: As a dietary supplement, mix 1-2 scoops (3-6g) with 8-12 ounces of cold water or juice. Consume 1-3 times daily between meals. As a pre-workout booster, consume up to 2 scoops 15-30 minutes before exercise.