Bis(pivaloyloxymethyl) azidothymidine 5'-monophosphate (piv 2-AZTMP) was designed as a cell membrane-permeable precursor of AZTMP. We have reported previously that when incubated with CEM cells deficient in thymidine kinase, piv 2-AZTMP gives rise to intracellular AZTMP and the corresponding diphosphate (AZTDP) and triphosphate (AZTTP). Under similar conditions, no intracellular nucleotides were formed with AZT. However, the mechanism bỹ ich piv 2-AZTMP is converted to AZTMP has not been established. To address this question, we have used the recently developed 'on-line ISRP-cleaning' HPLC technique to investigate the stability and metabolic fate of piv 2-AZTMP (1) in RPMI 1640 medium, (2) in RPMI containing 10% heat-inactivated fetal calf serum, and (3) in CEM cell extracts. Similar studies were conducted starting with mono(pivaloyloxymethyl) azidothymidine 5'-monophosphate (piv 1-AZTMP).From the kinetics of these reactions, it appears that piv 2-AZTMP is slowly hydrolyzed to piv 1-AZTMP in RPMI and that the metabolic sequence in cell extract and in tissue culture medium is clearly: piv 2-AZTMP~piv1-AZTMP~AZTMP~AZT. The rate constants are quite different in these three media. Although it is evident that the first step in the metabolism of piv 2-AZTMP is catalysed by carboxy-. *For correspondence. Tel. +67545873; Fax. +67 0420 29. late esterase, the enzyme(s) responsible for the second step, piv1-AZTMP~AZTMP, is less apparent, as carboxylate esterases and/or phosphodiesterases can be taken in account. However, analysis of the kinetic data strongly suggests that carboxylate esterase does not playa significant role and that this second step is mediated by phosphodiesterases. Collectively, these studies demonstrate, that piv 2-AZTMP is an effective prodrug of AZTMP. They also establish that piv1-AZTMP is an intermediate in this process, and define the sequence of the overall metabolic reaction. With this increased understanding of the metabolism of piv 2-AZTMP, it should be possible rationally to design analogues with optimal structural and pharmacological properties for use in vivo.