In isolated, perfused mouse livers, initial rates of uptake of [2-14C]pseudoisocytidine (PIC), measured during the first 15 secs of perfusion were markedly reduced when the perfusion medium contained 5 X 10(-6) M nitrobenzylthioinosine (NBMPR), a potent inhibitor of nucleoside transport. A similar inhibition of PIC uptake occurred when mice were treated with NBMPR-P (the 5'-monophosphate of NBMPR) at doses greater than 0.2 mg/kg ip injected 30 mins prior to the liver perfusion assay. However, in vivo studies showed that a late effect of NBMPR-P was enhancement in PIC levels in liver and other tissues in mice and rats, relative to levels in animals that had not received NBMPR-P. Increases in incorporation of PIC into RNA reflected the NBMPR-P-induced increases in tissue levels of PIC. NBMPR-P and other inhibitors of nucleoside transport may have therapeutic applications in manipulation of the pharmacokinetic behavior and toxicity of nucleoside drugs.