Rats were administered L-threonine in the diet at concentrations of 0 (basal diet control), 1.25%, 2.5%, or 5.0% for 13 weeks. Animals were sacrificed following the treatment period or after a 5-week recovery period (for animals receiving the control or 5.0% L-threonine diet). The mean achieved doses of L-threonine during the treatment period were 0, 811.5, 1615.3, and 3266.9 mg/kg body weight/ day in males, and 0, 909.9, 1850.0, and 3673.3 mg/kg body weight/day in females. No toxicologically significant changes in general condition, body weight, food consumption, feed efficiency, water intake, ophthalmoscopy, urinalysis, hematology, blood chemistry or pathology were observed. Based on the results of the study, no-observed-adverse effect levels (NOAEL) of 3266.9 and 3673.3 mg/kg body weight/day can be established for male and female rats, respectively, under the present experimental conditions.