The effects of adenosine 5′-triphosphate (ATP) and ATP analogs, P/sub 2y/ purinoceptor agonists, on growth of normal mouse mammary epithelial cells (NMuMG) were examined. Cells were plated onto 24 well plates in DMEM supplemented with 10 % fetal calf serum. After serum starvation for 24 hours, ATP, P/sub 2y/ purinoceptor agonists (AdoPP[NH]P, ATP-α-S, ATP-γ-S, β, γ-me-ATP and 2me-S-ATP), P/sub 2u/ purinoceptor agonist (UTP) and P/sub 2y/ purinoceptor antagonists (Reactive Blue 2, more selective to P/sub 2y/ receptor than PPADS; PPADS) were added. DNA synthesis was estimated as incorporation of 3H-thymidine into DNA (1 hour pulse with 1 μ Ci/ml, 18～19 hours after treatment). ATP, Adopp[NH]P, ATP-α-S or ATP-γ-S, significantly increased DNA synthesis at 1, 10 and 100 μM concentrations with dose-dependency (P<0.05), and the maximum responses of ATP and ATP analogs were shown at 100 μM concentration (P<0.05). The potency order of DNA synthesis was ATP≥ATP- γ -S>Adopp [NH]P>ATP-α-S. β, γ -me-ATP, 2me-S-ATP and UTP did not increase DNA synthesis. In autoradiographic analysis of percentage of S-phase cells, similar results were observed to those of DNA synthesis. Addition of 1, 10 or 100 μM Reactive Blue 2 or PPADS significantly decreased ATP (100 μM)-induced DNA synthesis, however, PPADS was less effective than Reactive Blue 2. In Elvax 40P implant experiment, ATP directly stimulated mammary endbud growth in situ suggesting the physiological regulator of ATP in mammary growth. ATP 100 μM rapidly increased MAPK activity, reaching a maximum at 5 min and then gradually decreasing to the base level in 30 min. ATP analogs, Adopp[NH]P and ATP-γ-S also increased MAPK activity, however, β, γ-me-ATP and 2me-S-ATP did not. The inhibitor of the upstream MAPK kinase (MEK), PD 98059 (25 μM), effectively reduced ATP (100 μM) or EGF(10 ng/ml, as positive control)-induced MAPK activity and DNA synthesis (P<0.05). These results indicate that ATP-induced DNA synthesis was prevented from the direct inhibition of MAPK kinase pathway. Overall results support the hypothesis that the stimulatory effects of normal mouse mammary epithelial growth by addition of ATP or ATP analogs are mediated through mammary tissue specific P/sub 2y/ purinoceptor subtype, and MAPK activation is necessary for the ATP-induced cell growth.