In the present study we have explored the catabolism of dCyd and 2',3'-dideoxycytidine (ddC) in mature monocyte-derived macrophages (M/M) and focused on its relation to the accumulation of intracellular anabolites. We found that dCyd catabolism in M/M was significant in 1-week old cultures (15% of 0.5 microM dCyd, 0.5 nmole/million cells, catabolized within one hour of incubation) and further increased in more mature 3-week and 5-week old cultures (> 85% catabolized within one hour). Dihydrouracil (DHU) was irreversibly formed as end product of the dCyd catabolism, with dUrd and Ura as intermediate metabolites. Intracellularly, dCyd nucleotides were detected up to three hours of incubation, with dCTP maintained at steady levels between one and three hours. However, after twelve hours of incubation DHU was the only detectable intracellular as well as extracellular metabolite. In contrast to what was observed with dCyd, we found that ddC was resistant to catabolism, with no detectable catabolites formed within twenty-four hours of incubation. Therefore ddC anabolism was unopposed by catabolic pathways and consequently intracellular anabolites of ddC accumulated throughout twenty-four hours of incubation. Based on these results, we propose that lack (or inhibition) of catabolism is at least as important as efficient anabolic phosphorylation for obtaining therapeutic effects of pyrimidine nucleoside analogs.