The effects of medullary NO on Na excretion are reduced in Dahl salt-sensitive (SS) compared to salt-resistant rats (SR). We have shown that flow-induced NO production by thick ascending limbs (THALs) of SS is reduced when rats are on a normal salt diet although NO synthase 3 protein expression was the same. In Sprague-Dawley rats a high-salt diet markedly increase NOS3 expression. It is unknown whether this is also the case for both SS and SR fed a high-salt diet. We hypothesized that flow-induced NO production by SS THALs is blunted in part due to a failure to increase NOS3 expression in response to dietary salt. SS and SR were fed normal and high-salt diets for 7-9 days. We measured systolic blood pressure (SBP); flow-induced NO in isolated THALs; and NOS3 protein expression in renal medullary lysates. SBP was 164.5±3.2 mm Hg ( n = 9) in SS and 130.1±4.8 in SR when fed a high-salt diet ( n = 7 ; p < 0.0001 ) . Flow-induced NO production by SS THALs from rats fed high salt was 9 ± 2 arbitrary units (AU)/min ( n = 6) while it was 44 ± 10 AU/min by SR tubules ( n = 6), significantly greater ( p < 0.005). NOS3 expression was 0.74 ± 0.08 AU ( n = 5) in THALs from SS on high salt while it was 1.26 ± 0.08 AU in tubules from SR ( n = 5), significantly greater ( p < 0.003). THAL NOS3 expression was unchanged by a high-salt diet in SS (1.01 ± 0.08 vs 1.11 ± 0.11; n = 5) but increased in SR. We conclude that decreased flow-induced NO production in high-salt-fed SS is at least partially due to failure to increase NOS3 expression.