The A1-adenosine receptor (A1-AR) is a member of the G-protein coupled receptor superfamily, which has significant pathophysiological importance in disorders such as heart arrhythmias, asthma and stroke. Here, we have used fluorescence correlation spectroscopy (FCS) to facilitate the study of A1-AR pharmacology at the subcellular level. To this end, we have successfully designed and synthesised a fluorescently labelled A1-AR agonist, ABA-BY630. ABA-BY630 is an N6- derivative of adenosine conjugated to the red-excited fluorophore, BODIPY" 630/650. In CHO cells expressing the human A1-AR, ABA-BY630 shows strong and potent agonist activity at this receptor. Specific binding of ABA-BY630 to the A1-AR in cell membranes of living CHO cells can also be visualised using confocal microscopy. Moreover, using FCS, we can detect and quantify the binding of ABA-BY630 to the A1-AR in a small area (0.2 microm2) of the upper cell membrane. FCS measurements indicate the presence of at least two populations of receptor-ABA-BY630 complexes with diffusion times of 8 and 233 ms. The quantity of both of these complexes was significantly reduced by pre-incubation with the A1-AR antagonist DPCPX. Application of FCS in conjunction with ABA-BY630 will allow the comparison of A1-AR pharmacology in single cells from healthy and diseased tissue.