Novel as well as known 5'-N-substituted carboxamidoadenosines were prepared via new routes that provided shorter reaction times and good yields. Binding affinities were determined for rat A1 and A2A receptors and human A3 receptors. EC50 values were determined for cyclic AMP production in CHO cells expressing human A2B receptors. On all receptor subtypes relatively small substituents on the carboxamido moiety were optimal. Selectivity for the A3 receptor was found for several analogues (1a, 1d, 1h, and 1k). On A1 receptors a number of compounds, but not 5'-N-ethylcarboxamidoadenosine (NECA, 1b), showed small GTP shifts, which could be indicative of lower intrinsic activities at the A1 receptor. At the A2B receptor, derivatives 1i-k with modified ethyl substituents had reduced activities compared to the A2B reference agonist NECA (1b). Thiocarboxamido derivatives (8b and 8c) displayed considerable although decreased A2B receptor activity. The X-ray structure determination of compound 8b was carried out. Due to intramolecular hydrogen bonding between the carboxamido NH and the purine N3 in the crystal structure, the ribose moiety of this compound is in a syn conformation. However, theoretical calculations support that NECA (1b), and less so 8b, can readily adopt both the syn and the anti conformation, therefore not excluding the proposed anti mode of binding to the receptor.