The levels of ribavirin or its antivirally active metabolic products were determined in the serum and urine of rats treated with single oral doses of 1,000 or 100 mg/kg of the compound, using a newly developed micromethod in which measles virus inhibition was assayed in BS-C-1 cells. At the high dosage level, maximum ribavirin serum levels of 24 μg/ml were observed 2 h postribavirin administration. Approximately 10-fold less active material was seen in the rats receiving the lower ribavirin dosage; this peak effect was seen 1 h after treatment. Urine excretion was maximal between 4 and 20 h after treatment in both dosage groups