A series of & (substituted phenyl) derivatives of theophylline and other 1, 3-diakylxanthmes were evaluated for potency and selectivity as antagonists at A,-and A2-adenosine receptors in brain tissue. Theophylline has a similar potency (Ki= 14~ LM) at both A, and A2 receptors. 8-Phenyltheophylline is 25-35-fold more potent as an adenosine receptor antagonist than theophylline, while 8-phenylcaffeine is only 2-3-fold more potent than caffeine. A p- ...