The pharmacokinetics and tissue distribution of rat fibroblast interferon (RfIFN) after i.v., i.p. and i.m. administration were studied. RfIFN i.v. administered into Lewis rats was rapidly cleared from the blood with a half-life of about 10 min during the 1st hr. After 1 hr, clearance was slower. Intraperitoneal- and i.m.-injected RfIFN showed peak serum levels 1 hr after injection. Serum levels after i.m. administration were lower than after i.p. injection. However, no RfIFN was detected in the blood 24 hr after injection by any of these three routes. The injected RfIFN was found in kidney, liver, spleen and lung, but RfIFN levels were 3 to 10 times higher in the kidney than in any other organ. No RfIFN was found in the urine. Very low levels of RfIFN were found in the cerebrospinal fluid of normal rats. In rats with active experimental allergic encephalomyelitis, the amount of RfIFN in the cerebrospinal fluid was small but 3 times higher than in normal rats. No RfIFN was found in the brain tissue of normal or experimental allergic encephalomyelitis rats.