The A1adenosine receptor (A1ar) antagonist 1,3-dipropyl-8-( p-acrylic)phenylxanthine (BW-1433) was administered to lean and obese Zucker rats to probe the influence of endogenously activated A1ars on whole body energy metabolism. The drug induced a transient increase in lipolysis as indicated by a rise in serum glycerol in obese rats. The disappearance of the response by day 7 of chronic studies was accompanied by an increase in A1ar numbers. Glucose tolerance tests were administered to rats treated with BW-1433. Peak serum insulin levels and areas under glucose curves (AUGs) were 34 and 41% lower in treated obese animals than in controls, respectively, and 19 and 39% lower in lean animals. With chronic administration (6 wk), AUGs decreased 47 and 33% in obese and lean animals, respectively. There was no effect of BW-1433 in either lean or obese rats on weight gain or percent body fat. Thus the major sustained influence of whole body A1ar antagonism in both lean and obese animals was an increase in whole body glucose tolerance at lower levels of insulin.