There is growing evidence for the involvement of nitric oxide (NO) -mediated nitrosation in cell signaling and pathology. Although S-nitrosothiols (RSNOs) have been frequently implicated in these processes, it is unclear whether NO forms nitrosyl adducts with moieties other than thiols. A major obstacle in assessing the significance of formation of nitrosated species is the limited reliability of available analytical techniques for measurements in complex biological matrices. Here we report on the presence of nitrosated compounds in plasma and erythrocytes of rats, mice, guinea pigs, and monkeys under basal conditions, in immunologically challenged murine macrophages in vitro and laboratory animals in vivo. Besides RSNOs, all biological samples also contained mercury-stable nitroso species, indicating the additional involvement of amine and heme nitros(yl)ation reactions. Significant differences in the amounts and ratios of RSNOs over N- and heme-nitros(yl)ated compounds were found between species and organs. These observations were made possible by the development of a novel gas-phase chemiluminescence-based technique that allows detection of nitroso species in tissues and biological fluids without prior extraction or deproteinization. The method can quantify as little as 100 fmol bound NO and has been validated extensively for use in different biological matrices. Discrimination between nitrite, RSNOs, and N-nitroso or nitrosylheme compounds is accomplished by use of group-specific reagents. Our findings suggest that NO generation in vivo leads to concomitant formation of RSNOs, nitrosamines, and nitrosylhemes with considerable variation between rodents and primates, highlighting the difficulty in comparing data between different animal models and extrapolating results from experimental animals to human physiology.