A case of herpes encephalitis diagnosed by brain biopsy and treated with 5-iodo-2-deoxyuridine (IDU) is presented. The infection occurred in a previously well 19-year-old female patient. Plasma and cerebrospinal fluid (CSF) uptake of the substance was determined using 125I labelled IDU. Top CSF levels of IUD and metabolites of less than 4 microgram/ml, about 1/10 of the corresponding plasma level, were obtained after 6 hours of continuous infusion. The result is discussed and compared with a similar study made on 5 healthy beagle dogs where in addition the levels obtained in various parts of the brain were determined. In the animal experiment a mean value of 2.5 microgram/ml of IDU and metabolites was obtained in the CSF after 8 hours, less than 1/20 of the plasma level. The levels in brain tissue were only slightly higher than in the CSF. The causes of therapeutic failures with IDU treatment are discussed.