The objective of this study was to create constitutively active mutant human A, adenosine receptors (ARs) using single amino acid replacements, based on findings from other G protein-coupled receptors, A, ARs mutated in transmembrane helical domains (TMs) 1, 3, 6, and 7 were expressed in COS-7 cells and subjected to agonist radioligand binding and phospholipase C (PLC) and adenylyl cyclase (AC) assays. Three mutant receptors, A229E in TM6 and R108A and R108K in the DRY motif of TM3, were found to be constitutively active in both functional assays. The potency of the A, agonist CI-IB-MECA (1-chloro-N-6-(3-iodobenzyl)adenosine-5'-N-methyluronamide) in PLC activation was enhanced by at least an order of magnitude over wild type (EC50 951 nM) in R108A and A229E mutant receptors. CI-IB-MECA was much less potent (>10-fold) in C88F, Y109F, and Y282F and mutants or inactive following double mutation of the DRY motif. The degree of constitutive activation was more pronounced for the AC signaling pathway than for the PLC signaling pathway. The results indicated that specific locations within the TMs proximal to the cytosolic region were responsible for constraining the receptor in a G protein-uncoupled conformation.