The A 3 adenosine receptor (A3AR) is one of four receptor subtypes for adenosine and is expressed in a broad spectrum of tissues. In order to study the function of A3AR, a mouse line carrying a mutant A 3 allele was generated. Mice homozygous for targeted disruption of the A3AR gene, A3AR ؊/؊ , are fertile and visually and histologically indistinguishable from wild type mice. The lack of a functional receptor in the A3AR ؊/؊ mice was confirmed by molecular and pharmacological analyses. The absence of A3AR protein expression in the A3AR ؊/؊ mice was demonstrated by lack of N 6 -(4-amino-3-[ 125 I]iodobenzyl)adenosine binding to bone marrowderived mast cell membranes that were found to express high levels of A3AR in wild type mice. In A3AR ؊/؊ mice, the density of A 1 and A 2A adenosine receptor subtypes was the same as in A3AR ؉/؉ mice as determined by radioligand binding to brain membranes. Additionally, A 2B receptor transcript expression was not affected by ablation of the A3AR gene. A3AR ؊/؊ mice have basal heart rates and arterial blood pressures indistinguishable from A3AR ؉/؉ mice. Functionally, in contrast to wild type mice, adenosine and the A3AR-specific agonist, 2-chloro-N 6 -(3-iodobenzyl)-adenosine-5-N-methylcarboxamide (2-Cl-IB-MECA), elicit no potentiation of antigen-dependent degranulation of bone marrow-derived mast cells from A3AR ؊/؊ mice as measured by hexosaminidase release. Also, the ability of 2Cl-IB-MECA to inhibit lipopolysaccharide-induced tumor necrosis factor-␣ production in vivo was decreased in A3AR ؊/؊ mice in comparison to A3AR ؉/؉ mice. The A 2A adenosine receptor agonist, 2-p-(2-carboxyethyl)phenylamino)-5-Nethylcarboxamidoadenosine, produced inhibition of lipopolysaccharide-stimulated tumor necrosis factor-␣ production in both A3AR ؊/؊ and A3AR ؉/؉ mice. These results show that the inhibition in vivo can be mediated by multiple subtypes, specifically the A 3 and A 2A adenosine receptors, and A3AR activation plays an important role in both pro-and anti-inflammatory responses.Adenosine is a naturally occurring nucleoside that exhibits diverse and potent physiological responses in the central nervous system and the cardiovascular, renal, pulmonary, and immune systems. The actions of adenosine are mediated through G-protein-coupled receptors classified into four subtypes, A 1 , A 2A , A 2B , and A 3 , on the basis of their affinity order profiles for agonists and antagonists (1-3). The A 3 adenosine receptor (A3AR) 1 is the most recent subtype identified, since it had remained pharmacologically obscure until its gene was cloned in the early 1990s. The A3AR has been cloned from multiple species including rat (4, 5), human (6, 7), sheep (8), canine (9), and rabbit (10).The physiological importance of the A 3 adenosine receptor has not been established. A3AR is expressed in a broad spectrum of tissues. The most abundant expression in human is found in lung and aorta (7). The evaluation of the functional role of the A3AR has been hampered by the presence of multiple adenosine receptor s...