A large series of derivatives and analogues of the uracil nucleotides UMP, UDP, and UTP with modifications in various positions of the uracil moiety and/or the phosphate groups were synthesized and evaluated at human P2Y(2), P2Y(4), and P2Y(6) receptors. 2-(Ar)alkylthio substitution of UMP and UDP was best tolerated by the P2Y(2) receptor. 2-Phenethylthio-UMP (13e) showed an EC(50) value of 1.3 μM at P2Y(2) and >70-fold selectivity versus P2Y(4) and P2Y(6) receptors. Substitution of the 2-keto group in UMP by NH (13g, iso-CMP) resulted in the first potent and selective P2Y(4) agonist (EC(50) 4.98 μM, >20-fold selective vs P2Y(2) and P2Y(6)). In contrast, replacement of the 2-keto function in UDP by NH yielded a potent P2Y(2) agonist (12g, iso-CDP, EC(50) = 0.604 μM, >100-fold selective). In an attempt to obtain metabolically stable UTP analogues, β,γ-dichloro- and β,γ-difluoro-methylene-UTP derivatives were synthesized. The triphosphate modifications were much better tolerated by P2Y(2), and in some cases also by P2Y(6), than by P2Y(4) receptors. 4-Thio-β,γ-difluoromethylene-UTP (14g) was a potent P2Y(2) agonist with an EC(50) value of 0.134 μM and >50-fold selectivity. N3-Phenacyl-β,γ-dichloromethylene-UTP (14b) proved to be a potent P2Y(6) receptor agonist (EC(50) 0.142 μM) with high selectivity versus P2Y(4) (50-fold) and moderate selectivity versus P2Y(2) receptors (6-fold).