Hypoglycin A, the toxin found in the ackee fruit, has been reported in the literature as the causative agent in incidences of acute toxicity termed Jamaican vomiting sickness or toxic hypoglycemic syndrome. Hypoglycin A toxicity in this study was determined by feeding male and female Sprague-Dawley rats a control diet and ackee diets that contained 4-3840 ppm of hypoglycin. The fixed dose method was used to quantify the acute toxic dose of hypoglycin A and was determined by feeding a diet consisting of the lowest hypoglycin A concentration; this was increased to the next highest dose after 24h until toxicity was observed. The maximum tolerated dose (MTD) of hypoglycin A was determined by feeding rats the ackee and control diets over a 30-day period. The acute toxic dose for male and female rats was 231.19+/-62.5 5mg hypoglycinA/kgBW and 215.99+/-63.33 mg hypoglycinA/kgBW, respectively. This was considerably greater than the dose of 100 mg hypoglycin/kgBW reported in a previous study when aqueous hypoglycin was administered orally. The MTD of hypoglycin A in both male and female rats was 1.50+/-0.07 mg hypoglycinA/kgBW/day. These findings suggest that the form in which hypoglycin in ackee is administered could affect the toxicological properties it exhibits. Therefore, for the purpose of a hazard assessment, it may be best administered within the matrix of the fruit, which is the form that humans consume it.