Treatment of HL-60 cells with micromolar concentrations of N 6 -benzyladenosine (N 6 -benzylaminopurine riboside [BAPR]) led to the occurrence of apoptosis in a concentration-dependent manner. Incubation period as short as 2 h in the presence of BAPR was sufficient for triggering irreversible changes leading to apoptosis even after the transfer of cells to the standard medium (without BAPR). Cell death induced by BAPR proceeded rapidly and in a very synchronous fashion. Detailed study of temporal changes in a chromatin structure and DNA integrity showed that the movement of chromatin toward the nuclear periphery is the fundamental event within dying cells. We demonstrated that this rearrangement of chromatin is irreversible and it takes place without apparent DNA degradation. The extensive DNA cleavage seems to be a rather late event, as it was observed in cells that exhibited a typical apoptotic morphology (apoptotic bodies). On the basis of temporal changes in the ATP level within dying cells, it is concluded that ATP is essential for the movement of chromatin toward the nuclear envelope but not for the subsequent chromatin condensation leading to the formation of apoptotic bodies. DNA fragmentation also seems to be ATP independent. BAPR interfered with neither pyrimidine nor purine biosynthesis, as none of the tested bases and the corresponding nucleosides prevented or reduced apoptosis in BAPR-treated cells. Adenosine was the only exception that substantially reduced the effect of BAPR. Since transport of exogenous adenosine into cells was essential to manifest its protective effect, we assume that adenosine is a competitive inhibitor of adenosine kinase and thus reduces intracellular phosphorylation of BAPR. Indeed, 4-amino-3-iodo-1(-D-ribofuranosyl)pyrazolo [3,4-d]pyrimidine, a potent inhibitor of adenosine kinase, fully prevented BAPR-induced apoptosis. These results suggest that cytotoxic effect of BAPR is related to its activation by phosphorylation within cells, rather than to its interaction with extracellular adenosine receptors.