Bacterial nitric-oxide (NO) synthases (bNOSs) are smaller than their mammalian counterparts. They lack an essential reductase domain that supplies electrons during NO biosynthesis. This and other structural peculiarities have raised doubts about whether bNOSs were capable of producing NO in vivo. Here we demonstrate that bNOS enzymes from Bacillus subtilis and Bacillus anthracis do indeed produce NO in living cells and accomplish this task by hijacking available cellular redox partners that are not normally committed to NO production. These "promiscuous" bacterial reductases also support NO synthesis by the oxygenase domain of mammalian NOS expressed in Escherichia coli. Our results suggest that bNOS is an early precursor of eukaryotic NOS and that it acquired its dedicated reductase domain later in evolution. This work also suggests that alternatively spliced forms of mammalian NOSs lacking their reductase domains could still be functional in vivo. On a practical side, bNOS-containing probiotic bacteria offer a unique advantage over conventional chemical NO donors in generating continuous, readily controllable physiological levels of NO, suggesting a possibility of utilizing such live NO donors for research and clinical needs.