The objective of this study was to determine if adenosine alters growth of mammary epithelium. Mouse mammary epithelial cells (NMuMG) were cultured in DMEM supplemented with 10% fetal calf serum. After serum starvation for 24h, EGF (0–100ng/ml) and/or adenosine (0–100μm) was added. Adenosine at concentrations of 1, 10 or 100μmincreased DNA synthesis significantly, when compared to control. Addition of epidermal growth factor (EGF) (10ng/ml) into 1 or 10μmadenosine showed the interaction in DNA synthesis between EGF and adenosine. A similar result was observed when 100μmadenosine added to various concentrations of EGF (0–100ng/ml). In the second mammary gland (thoracic) organ culture studies, mammary development scores were increased by adenosine (100μm), EGF (100ng/ml) and adenosine plus EGF. These results indicate that the purine nucleoside adenosine stimulates mammary epithelial cell growth and interacts with EGF in DNA synthesis of mouse mammary epithelial cells.