New derivatives of PD 81,723, an allosteric enhancer of agonist binding to the A1-adenosine receptor, have been synthesized and evaluated in an intact cell assay. Compounds 3a, 3o and 3p appeared to be more potent than PD 81,723 and at a concentration of 0.1 microM caused significant reductions of cAMP content of CHO cells expressing the human A1-adenosine receptor. Compounds 4e and 4o appeared to be allosteric enhancers at a low concentration and antagonists at a higher concentration, whereas compounds 3c, 3g, 3s and 4l appeared to be weak antagonists that are also allosteric enhancers at the higher concentration of 10 microM.