ATP has been shown to act as a modulator in various functions of kidney. However, the effect of ATP on the proliferation of renal proximal tubule cells (PTCs) has not been elucidated. Therefore, this study investigated the effect of ATP on cell proliferation and its related signal pathways in primary cultured PTCs. ATP (>10^-5 M, 1 hr) was found to stimulate thymidine and BrdU incorporation. The ATP (10^-4 M)-induced stimulation of thymidine incorporation was blocked by suramin (a P2X and P2Y receptor antagonist), RB-2 (a P2Y receptor antagonist), MRS 2159 (a P2X1 receptor antagonist) or MRS 2179 (a P2Y1 receptor antagonist). Moreover, ATP increased [Ca²⁺]_i which was blocked by suramin, methoxyverapamil, and EGTA. The ATP-induced stimulation of cell proliferation was also blocked by EGTA (an extracellular calcium chelator), methoxyverapamil (a calcium antagonist), and nifedipine (a L-type calcium channel blockers), suggesting a role of calcium influx. In addition, the ATP-induced phosphorylation of p38 and p44/42 MAPKs was blocked by nifedipine. On the other hand, ATP increased the cyclin dependent kinase (CDK)-2, CDK-4 and cyclin E expression levels, which were blocked by suramin, RB-2, MRS 2179, MRS 2159 or nifedipine. However, ATP decreased the p21 WAF1/Cip1 and p27 kip1 expression levels. The ATP-induced stimulation of thymidine incorporation and increase of CDK-2 and CDK-4 expression were blocked by SB 203580 (a p38 mitogen activated protein kinase [MAPK] inhibitor) and PD 98059 (a MEK inhibitor) but not by SP 600125 (a JNK inhibitor). In conclusion, ATP stimulates proliferation by increasing [Ca²⁺]_i, and activating p38, p44/42 MAPKs and CDKs in the PTCs.