(E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) was found by HPLC analysis to be rapidly metabolized in mice and in liver homogenates from mouse and man to the antivirally inactive (E)-5-(2-bromovinyl) uracil (BVU) but was comparatively stable in blood from both species. Of a series of 5'-O-alkoxycarbonyl derivatives of BVDU, the 5'-O-tert.-butoxycarbonyl derivative (BRL 37000) was the most stable in mouse and human blood and liver homogenates, neither its ester bond nor its N-glycosidic linkage being readily cleaved enzymically. Oral administration of BRL 37000 and the 5'-O-ethoxycarbonyl derivative (BRL 36101) to mice gave prolonged serum concentrations of BVDU and delayed the appearance of BVU compared with the BVDU control. BRL 36101 was more active than BVDU when administered orally to mice infected cutaneously with herpes simplex virus type 1.