The hypothesis that purine nucleotides and nucleosides affect brown fat preadipocyte proliferation was tested using isolated rat interscapular brown fat preadipocytes in culture. Daily addition of 100 microM adenosine triphosphate (ATP) (n = 4) to cultures enhanced the relative DNA content by 1.5-fold compared to control cultures (P < 0.05) measured using CyQUANT-GR fluorescence. Higher concentrations of ATP inhibited growth and 500 (n = 2) or 1000 microM ATP (n = 3) almost completely inhibited growth. ATP (100 microM) did not affect while 250-1000 microM ATP decreased protein content relative to control cultures. Adenosine (100 microM; n = 3) did not affect DNA or protein content, but 500 microM and 1000 microM adenosine suppressed brown adipocyte proliferation and inhibited protein synthesis. Cultured brown adipocytes quickly removed or degraded ATP in the culture media as determined by luciferin-luciferase bioluminescence, suggesting that the inhibitory effects of high ATP concentrations may result from its breakdown to adenosine. The results support the conclusion that ATP promotes and adenosine inhibits brown adipocyte proliferation.