already used compound 7 to mask out the Al component of [3H]NECA binding in tissues such as dog or human brain where CPA does not appear to be sufficiently selective for this purpose. Separation by cyclodextrin HPLC of the individual diastereomers of 7 afforded compounds 8 and 9 (compound 9 eluted before 8). Compound 8 is the most potent (Ki = 0.24 nM) and selective (16000-fold) agonist for the Al receptor to be reported to date. As anticipated, the other diastereomer (9) is less active and less selective for the Al receptor. In summary, in this study we have identified novel N6-[2.2.1 ]bicycloalkyladenosines with unusually high potency and selectivity for the adenosine Al receptor. Compounds 4, 7, and 8 should serve as important tools for further characterization of subpopulations of adenosine receptor subtypes in various tissues.