An in-parallel comparison is presented of the in vitro and in vivo properties of two 9-deazaguanine analog inhibitors of purine nucleoside phosphorylase (PNP), CI-972 [8-amino-9-deaza-9-(3-thienylmethyl)guanine] and PD 141955 [9-deaza-9-(3-thienylmethyl)guanine] (published Ki values of 0.83–8.0 and 0.08 μM, respectively). Despite structural similarities, PD 141955 was considerably more potent and active in all systems studied. The respective ic50 values for inhibition of MOLT-4 cell growth in the absence and presence of 10 μM 2'-deoxyguanosine (GdR) were > 50 and 5.06 μM for CI-972 and 15.4 and 0.061 μM for PD 141955. PD 141955 induced accumulation of dGTP in GdR-treated MOLT-4 and CEM cells at log-lower concentrations than were required of CI-972, and the magnitude of dGTP accumulation in PD 141955-treated T cell cultures was markedly greater (e.g. 366 vs 100 pmol106 CEM cells at 10 μM). PD 141955 administered orally produced a dose-dependent elevation of plasma inosine and guanosine in rats over a broad concentration range. Mean plasma inosine concentrations following a 150 mgkg p.o. dose peaked at 6.21 and 13.2 μM in CI-972 and PD 141955-treated rats, respectively. Low levels of inosine were detectable at 50 μgkg following oral administration of PD 141955.