One of these pivotal studies involved oral dosage of aspartame to infant Rhesus monkeys for 52 weeks.
The research was conducted by the University of Wisconsin Medical Center in Madison, Wisconsin. A low dose group which received 1.0 g of aspartame/kg of body weight per day, a medium dose group receiving 3.0g/kg per day and a high dose group receiving 4-6 g/kg per day.
The high dose group ended up ingesting about the same amount as the medium dose group as the high dose monkeys would not consume intended levels of aspartame possibly because it was too sweet at that amount. The monkeys in this study were served their aspartame in an orally consumed milk based formula.
Starting about 7 months (218 days) into the experiment, ALL the medium and high dose monkeys began having brain seizures.
Testing of the monkeys in this study showed that there was in fact phenylalanine (which comprises 50% of aspartame) in their blood which proves that it is absorbed.