Introduction: Elevated serum phosphate is an independent risk factor for cardiovascular disease although the mechanism of action is unclear. This study looks at the effect of altered phosphate concentration in rat resistance vessels and rat smooth muscle (SMCs) and endothelial cells (ECs). Methods: 12 week old male WKY rats were sacrificed (schedule 1). 3rd order resistance vessels were dissected from the mesentery and incubated at 4°C for 16 hours in physiological saline solution (PSS) with normal (1.18mM) or high(2.5mM) phosphate concentration. Vessels were mounted on a wire myograph, normalized and assessed for the presence of an intact endothelium. Vasoconstriction response to increasing concentrations of phenylepherine (PE) and vasorelaxation response to increasing concentrations of carbachol were measured in the presence and absence of a phosphodiesterase 5 inhibitor, zaprinast. Vessels were then pre-constricted a second time with PE, again in the presence and absence of zaprinast, and vasorelaxation response to sodium nitroprusside (SNP) was measured. Concentration-response curves were constructed for each vessel for PE, carbachol and SNP +/- zaprinast. Area under the curve was calculated for each vessel and comparisons were made between the vessels in high and normal phosphate PSS and +/- zaprinast using a Student’s t test in SPSS. SMCs and ECs were grown in normal (0.5mM) and high (3mM) phosphate medium and used at passage 6-8. Cell lysates were obtained and Western blot was performed for total and phospho eNOS expression. Calcium concentration was measured in the cells by epifluoresence with FURA 2 AM. Results: Vessels in high phosphate relax less well in response to carbachol and SNP than those in normal phosphate (p<0.001 and p<0.05; Figure 1A and B). The contractile response to PE was similar and unaltered by the addition of zaprinast. The addition of zaprinast significantly improved the relaxation response of vessels to carbachol and SNP in both high and normal phosphate concentration PSS. This was most marked in the vessels in high phosphate (p<0.05) resulting in no significant difference between the relaxation response to carbachol and SNP between vessels in normal and high phosphate (Figure 1C). Expression of eNOS was lower in ECs but unchanged in SMCs grown in high phosphate compared with those in normal phosphate. Intracellular Ca2+ concentration was not significantly different in cells in high phosphate medium (228.3±21.97nM vs 278±22.44nM). Conclusion: Elevated phosphate decreases both endothelium dependent and independent vasorelaxation, possibly secondary to altered cGMP signaling. These effects seem to be independent of changes in calcium. These experiments mimic a uraemic state and may offer an explanation for elevated serum phosphate as a cardiovascular risk factor.