Instillation of diesel exhaust particles (DEP) does not affect endothelium-dependent relaxation, but impairs vasodilator response to sodium nitroprusside, in vivo. Percent change in femoral vascular conductance (FVC) from baseline in response to intra-arterial acetylcholine (ACh; 0.07 & 0.7 μg) and sodium nitroprusside (SNP; 0.09 & 0.9 μg) 6 h (a &c) and 24 h (b &d) after instillation. Responses were obtained in non-instilled rats (open column), saline-instilled rats (solid column) or DEP-instilled rats (0.5 mg; hatched column). Results are expressed as mean ± SEM (n = 4-6). *P < 0.05, **P < 0.01 DEP versus saline; two-way ANOVA followed by Bonferroni post-hoc test.