Adenosine (Ado, 10-50 microM), a potent inhibitor of ADP-induced human platelet aggregation in platelet-rich plasma (PRP), does not inhibit aggregation in whole blood. However, the Ado analogs, 2-fluoroadenosine, 2-chloroadenosine and 5'-N-ethylcarboxamidoadenosine (NECA) which are resistant to deamination (2-fluoroadenosine) or deamination and phosphorylation (2-chloroadenosine and NECA), inhibit aggregation in whole blood with IC50 values of 12 microM, 2.3 microM and 0.26 microM, respectively. The inhibitory effect of NECA (200 nM) is potentiated by the platelet cAMP phosphodiesterase (PDE) inhibitor RA 233 (5 microM). Inhibition of the erythrocytic nucleoside transport system by dilazep (1 microM) or dipyridamole (10 microM), or blockade of Ado metabolism by 2'-deoxycoformycin (5 microM) plus 5-iodotubercidin (10 microM), evokes the antiaggregatory action of Ado in whole blood (IC50 congruent to 2 microM). RA 233 (5 microM) potentiates Ado-mediated inhibition about 10-fold when nucleoside transport or Ado metabolism is blocked. Ado (10 microM or 200 nM) is rapidly metabolized within 1 min in whole blood. When nucleoside transport is inhibited by dilazep or dipyridamole, or when Ado metabolism is blocked by 2'-deoxycoformycin and 5-iodotubercidin, 50-60% of the Ado remains in the plasma after 5 min. These results show that the failure of Ado to inhibit platelet aggregation in whole blood results from its rapid uptake and metabolism by erythrocytes. More importantly, these data emphasize the key role of nucleoside transport inhibition in the antiplatelet actions of dipyridamole and dilazep. In addition, superior therapeutic results may be obtained from the combination of blockade of nucleoside transport system with inhibition of platelet cAMP PDE.