Adenosine triphosphate (ATP) has been shown to be an inhibitory or a stimulatory agent for cell growth in various types of cells. Here, we studied the effects of extracellular ATP on two pancreatic cancer cell lines, PK-1 and YAPC established by us. In both cell lines, ATP inhibited cell growth in a time- and dose-dependent manner, whereas the same doses of ATP stimulated DNA synthesis. Flow cytometric analysis of the cells incubated with or without ATP demonstrated the ATP-induced striking increase in cells at S-phase. The same analysis showed also the increase in sub-G0/G1 population in the same analysis and the electrophoretic pattern of DNA showed the occurrence of ATP-induced cell disintegration likely to be apoptosis. We suggest that extracellular ATP is cytotoxic for pancreatic cancer cells because of its induction of cell cycle arrest at S-phase and cell death, possibly apoptosis, overcoming the promotion of the entry into S-phase.