A complete understanding of the biological activity of nitric oxide (NO) is complicated by the different reactivity profiles of its various species and by the often complex decomposition behavior of the NO progenitors in common use. Here, we report that appropriately substituted C-nitroso compounds act solely as donors of neutral nitric oxide through a first-order homolytic C-N bond scission to release up to 88% nitric oxide in DMSO at 25 degrees C. The reaction produces a carbon radical, and the yield of nitric oxide is dependent on the availability of radical traps. C-Nitroso compounds are sources of biologically active neutral NO and display potent NO bioactivity in a rabbit aortic ring assay.