2-Phenylethynyladenosine and its N6-methyl derivative were synthesized and evaluated in binding assays at human adenosine receptors stably transfected on CHO cells. Results showed that the N6-methyl-2-phenylethynyladenosine is endowed with very high affinity and selectivity at A3 receptor subtype. Hence, an alternative procedure for the synthesis of tritiated N6-methyl-2-phenylethynyladenosine was set up to introduce tritiated methylamine in the final step.