The nature and potency of theophylline as a presynaptic adenosine receptor antagonist was investigated in rat vas deferens in vitro.Schild plots were constructed and the pA2 and the slope were determined when the presynaptic adenosine receptor agonist was (a) adenosine alone, (b) adenosine following adenosine uptake inhibition by hydroxynitrobenzylthioguanosine, and (c) 2-chloroadenosine, a potent adenosine analogue which appears not to be a substrate for the adenosine uptake system. The pA2 values for theophylline were 3.80, 4.58, and 5.61, respectively, and the slopes of the Schild plots were 0.58, 0.71, and 0.78, respectively.The variation in the apparent potency of theophylline as a presynaptic adenosine receptor antagonist is explained according to Furchgott's prediction that removal of an agonist from the vicinity of its receptor upsets the equilibrium between agonist and receptor and so may influence antagonist – receptor interactions.Because adenosine uptake varies among tissues, this process should be eliminated before attempting to compare potencies of adenosine receptor antagonists or to classify adenosine receptors by pA2 values.