A nonfatal case of accidental prolonged overdosage of pyrimethamine (10 times the usual dose for 10 days) in a seven-week-old infant treated for congenital toxoplasmosis is presented. The drug was identified and quantitated in plasma and urine using an HPLC technique with diode-array detection. Initial plasma concentration of pyrimethamine was 6.22 micrograms/mL. Results are discussed in light of the existing literature.