Nitric oxide $($NO$)$ is a cell$-$signaling agent in physiological processes including vasodilation, inhibition of clotting, and inflammation. A sensitive chromatography$-$mass spectrometry method was developed to measure two of its metabolites, nitrite $($NO$,)$ and nitrate $($NO;$),$ in biological fluids. Internal standards, $15$NO$5$ and $15$NO$5,$ were added to the fluid at concentrations of $75\mathrm{.}0$ and $750\mathrm{.}0$ uM$,$ respectively. Natural $4$NO$,$ and NO; plus the internal standards were then converted to volatile derivatives: