Treatment of isolated bone cells with adenosine increased cyclic 3’, 5’-AMP levels markedly but failed to modify total ATP levels. 100 μM-500 μM adenosine elicited maximal (17-fold), and 5 μM minimal(+68%), effects. Equivalent concentrations of 2’-AMP or 5’-AMP were less active than adenosine; and adenine, guanosine and uridine were inactive. The effect of adenosine peaked in 15-45 sec, gradually diminished with more prolonged treatment, and disappeared almost completely when treated cells were washed and incubated for an additional 30 sec in adenosinefree medium. These results suggest that adenosine acts at or near the cell surface to stimulate adenylate cydase. Adenosine and either sodium fluoride, epinephrine, or PTH increased bone cell cyclic 3’, 5’-AMP levels synergistically; and a physiological concentration of PTH (1 ng/ml) was highly effective in the presence of adenosine. Simultaneous treatment with theophylline decreased the effect of adenosine, yet theophylline did not inhibit the cellular accumulation of adenosine-8-14C. Hence, adenosine may also modify the responsiveness of adenylate cyclase to various stimuli, inhibit cyclic 3’, 5’-AMP phosphodiesterase activity, or both.