Adenosine 3',5'- and 2',5'-bisphosphates previously were demonstrated to act as competitive antagonists at the P2Y1 receptor (Boyer et al. Mol. Pharmacol. 1996, 50, 1323-1329). 2'- and 3'-Deoxyadenosine bisphosphate analogues containing various structural modifications at the 2- and 6-positions of the adenine ring, on the ribose moiety, and on the phosphate groups have been synthesized with the goal of developing more potent and selective P2Y1 antagonists. Single-step phosphorylation reactions of adenosine nucleoside precursors were carried out. The activity of each analogue at P2Y1 receptors was determined by measuring its capacity to stimulate phospholipase C in turkey erythrocyte membranes (agonist effect) and to inhibit phospholipase C stimulation elicited by 10 nM 2-MeSATP (antagonist effect). Both 2'- and 3'-deoxy modifications were well tolerated. The N6-methyl modification both enhanced antagonistic potency (IC50 330 nM) of 2'-deoxyadenosine 3',5'-bisphosphate by 17-fold and eliminated residual agonist properties observed with the lead compounds. The N6-ethyl modification provided intermediate potency as an antagonist, while the N6-propyl group completely abolished both agonist and antagonist properties. 2-Methylthio and 2-chloro analogues were partial agonists of intermediate potency. A 2'-methoxy group provided intermediate potency as an antagonist while enhancing agonist activity. An N1-methyl analogue was a weak antagonist with no agonist activity. An 8-bromo substitution and replacement of the N6-amino group with methylthio, chloro, or hydroxy groups greatly reduced the ability to interact with P2Y1 receptors. Benzoylation or dimethylation of the N6-amino group also abolished or greatly diminished the antagonist activity. In summary, our results further define the structure-activity of adenosine bisphosphates as P2Y1 receptor antagonists and have led to the identification of the most potent antagonist reported to date for this receptor.