Abstract Body: Exogenous serotonin (5-hydroxytryptamine, 5-HT) causes a reproducible decrease in blood pressure in normotensive and hypertensive rats. This is associated with dilation of arterioles in skeletal muscle. The 5-HT7 receptor KO rat strain allowed us to test the role of this receptor in both responses as well as the effects of endogenous 5-HT -- acting at 5-HT7 receptors -- on normal circulatory regulation. First, we hypothesized that the 5-HT7 receptor KO rat would have a higher hindquarter (HQ) vascular resistance (HQVR) than wildtype (WT) because of loss of this receptor. In rats anesthetized with isoflurane, instrumented with arterial catheters and flow probes on the terminal aorta, male KO rats had significantly higher resting HQVR [mm Hg/ml/min; KO (16.0+2.0) vs WT (10.8+0.6.0), p = 0.04] and lower HQ blood flow than normal Sprague-Dawley (SD) rats. No differences in resting HQVR or HQ flow were observed in female rats. Intravenous infusion of 5-HT (50 ug/kg/min) reduced systemic blood pressure and HQVR; these parameters were significantly attenuated (~56%) in male KO rats versus WT and SD animals. In females, falls in systemic blood pressure and HQVR were absent in both KO and WT rats. Second, as 5-HT has been implicated in the pathophysiology of heart failure (HF), a left anterior descending artery ligation model of HF was used to investigate the potential role of 5-HT7 regulation of HQ flow in HF-associated exercise intolerance. The ability of hindlimb muscle stimulation to increase blood flow using transdermal neuromuscular electrical stimulation was tested (NMES). M and F KO rats showed a reduced ability to increase HQ flow during muscle contraction vs WT (% over basal: M WT = 118+18 vs KO =14.6+7.1; F WT= 101+12 vs KO = 7.6+6) rats. This was observed in both intact and HF rats (in which flow changes were already impaired). Acute administration of the 5-HT7 receptor antagonist SB269970 abolished the ability of NMES to increase HQ flow in normal SD rats. Importantly, the microcirculatory capacity, visualized by Lectin-594, was not significantly different in the gastrocnemius muscle of WT (arbitrary fluorescence units/area: 946+40) vs KO (914+69). These data support that the 5-HT7 receptor, likely activated by endogenous 5-HT, is an important regulator of skeletal muscle vascular resistance. Thus, 5-HT may have a larger role in normal and pathophysiological regulation of the circulation than has previously been appreciated.