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Aspartame (L-aspartyl-L-phenylalanine methyl ester) is an esterified, dipeptide sweetener that is rapidly and completely metabolized in the gastrointestinal tract to phenylalanine, aspartic acid and methanol. The pharmacokinetics of phenylalanine (PHE) and tyrosine (TYR) were examined following the administration of oral doses of aspartame (APM) to fasted male Sprague-Dawley rats (0, 50, 100, 200, 500 and 1,000 mg/kg) and CD-1 mice (0, 100, 200, 500, 1,000 and 2,000 mg/kg). Peak plasma PHE/large neutral amino acid (LNAA) ratios were calculated. Maximal plasma PHE and TYR concentrations were observed within 1 h after dosing and returned to baseline within 4-8 h in both species regardless of the dose of APM. Mean PHE Cmaxs ranged from 73.6 to 1,161 nmol/ml in the rat, and from 78.6 to 1,967 nmol/ml in the mouse. TYR Cmaxs ranged from 91.6 to 502 nmol/ml and from 89.2 to 792 nmol/ml in the rat and mouse, respectively. AUCs and Cmaxs were linear with dose in both species. Peak plasma PHE/LNAA ratios ranged from 0.112 to 1.117 in rats and from 0.121 to 1.769 in mice. Comparison of these ratios with those observed previously in humans indicates that rodents require a 2-6 times higher dose of APM than humans to produce similar increases in plasma PHE/LNAA ratios.