This study investigated the role of changes in the expression of the cytochrome P450 4A (CYP450 4A) enzymes that produce 20-hydroxyeicosatetraenoic acid (20-HETE) in modulating the responses of rat mesenteric resistance arteries to norepinephrine and reduced PO₂ following short term (3 days) changes in dietary salt intake. The CYP450 4A2, 4A3 and 4A8 isoforms were all detected by RT-PCR in arteries obtained from rats fed high salt (HS) diet (4% NaCl), while only the CYP450 4A3 isoform was detected in vessels from rats fed low salt (LS) diet (0.4% NaCl). Expression of 51 kD CYP450-4A protein was significantly increased by HS diet. Inhibiting 20-HETE synthesis with 30 µM N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS) reduced the vasoconstrictor response to norepinephrine (NE) in arteries obtained from rats fed either a LS or HS diet, but NE sensitivity following DDMS treatment was significantly lower in vessels from rats on HS diet. DDMS treatment also restored the vasodilator response to reduced PO₂ that was impaired in arteries from rats on HS diet. These findings suggest that: 1) HS diet increases the expression of CYP450 4A enzymes in the mesenteric vasculature, 2) 20-HETE contributes to the vasoconstrictor response to norepinephrine in mesenteric resistance arteries, 3) the contribution of 20-HETE to the vasoconstrictor response to norepinephrine is greater in rats fed a HS diet than in rats fed a LS diet, and 4) upregulation of the production of 20-HETE contributes to the impaired dilation of mesenteric resistance arteries in response to hypoxia in rats fed a HS diet.