Methemoglobinemia has been reported in a small number of infants after topical application of benzocaine. Since it is a widely used medication, these rare occurrences have been considered to possibly represent idiosyncratic response. Varying amounts of benzocaine (1.5,3,6,12.5,25,50,100 mg) were administered by stomach tube, in a single 1.0 ml bolus, to 117 young (approx. 175 gm) Charles River CD male rats. Animals were sacrificed after 30 or 60 minutes and methemoglobin levels determined spectrophotometrically. Methemoglobin levels were negligible at the lowest dose of benzocaine (1.5 mg), but significant levels (20 - 70% of total hemoglobin) were found at the other dose levels. Pretreatment of animals with ascorbate (30 - 60 minutes before administering benzocaine) or simultaneous administration of varying amounts of ascorbate (up to 100 mg) failed to provide a protective effect during the period of observation (60 minutes) in animals receiving 3 or 6 mg benzocaine. Benzocaine does not induce methemoglobin formation in either incubated washed red cells or solutions of hemoglobin. Thus,the rapid formation of methemoglobin in intact animals suggests that the oxidant effect is caused by a metabolite of benzocaine. Washed red cells incubated in mixtures containing liver microsomes, benzocaine, and either NADH or NADPH were found to readily form methemoglobin, confirming notion that the oxidant is a biotransformation product of benzocaine. The relative rarity of benzocaine induced methemoglobinemia may be explained by overdosage rather than idiosyncratic response.