Perivascular adipose tissue secretes adipocyte-derived relaxing factor (ADRF) to relax mesenteric arteries by opening of smooth muscle delayed-rectifier K+ (Kv) channels in rats. The nature of ADRF is unknown. We studied adiponectin gene-deficient (Apn 1-/-) mice and tested the hypothesis that adiponectin plays a role in the paracrine control of vascular tone by perivascular adipose tissue. In mouse mesenteric arteries, recombinant adiponectin at serum levels (2.5 mg/mL) inhibited serotonin-dependent contractions. The effects were abolished by inhibition of Kv channels with 4-aminopyridine (4-AP, 2 mM). To study vascular function in Apn 1-/- mice, the mesenteric bed was cannulated and perfused at a constant flow of 4-5 mL/min in the absence and presence of serotonin. Inhibition of Kv channels with 4-AP (2 mmol/L) induced a similar increase in perfusion pressure in MB of Apn 1-/- mice, compared to wild-type mice. The anti-contractile effects of perivascular fat were similar in mesenteric artery and aortic rings from Apn 1-/- mice and wild-type mice. Our data indicate that similar to the rat, perivascular adipose tissue of the mouse harbors an ADRF. Adiponectin functions as a novel potent humoral vasodilator, which relaxes arterial rings by opening Kv channels. However, adiponectin does not play a role in the paracrine control of vascular tone by perivascular adipose tissue. Thus, the ADRF that we seek is not adiponectin.