The results of previous inhibitor studies suggested that there is some increase in nitric oxide (NO) production from constitutive NO synthase in early hemorrhage, but the magnitude of NO production early after hemorrhage has not been previously assessed. It is generally believed that only modest production rates are possible from the constitutively expressed NO synthases. To study this, anesthetized male Sprague-Dawley rats were subjected to 90 minutes of isobaric (40 mmHg) hemorrhage (H). During this period of time, the dynamics of accumulation of NO-intermediates in the arterial blood was assessed using electron paramagnetic resonance (EPR) spectroscopy, chemiluminescence, fluorescence imaging and mass spectrometry. EPR detectable NO-adducts were also measured with spin traps in blood plasma and RBCs. H lead to an increase in the concentration of hemoglobin-NO from 0.9 ± 0.2 to 4.8 ± 0.7 µM. This accumulation was attenuated by a non-selective inhibitor of NOS, L-NAME but not by D-NAME or 1400W. Administration of L-NAME (but not 1400W or D-NAME) during H produced a short-term increase in MAP (~90%). In H the level of N-oxides in RBCs increased 7-fold. S-nitrosylation of plasma proteins was revealed with "biotin switch" techniques. The results provide compelling evidence that there is brisk production of NO in early H. The results indicate that the initial compensatory response to H is more complicated than previously realized and it involves an orchestrated balance between intense vasoconstrictor and vasodilatory components.