We created a simple method based on curve fitting in order to assess the concentration of pharmacological agonists or antagonists in the microenvironment of the receptors. We tested our method in electrically driven guinea pig left atria by estimating the concentration of N6-cyclopentyladenosine (CPA; A1 adenosine receptor agonist), acetyl-beta-methylcholine (muscarinic receptor agonist) and verapamil (L-type Ca2+ channel inhibitor) added previously to the atria in known amounts. Our results validated the fitness of the model under specified conditions. In addition, our data suggest a relatively slow elimination of CPA in isolated, practically bloodless guinea pig atrial myocardium.