Nitric oxide, a gaseous free NO radical (.NO) generated in particulate-free gas-phase main-stream smoke of cigarettes, was observed with electrical spin resonance (ESR) using a spin trapping technique. N-Methyl-D-glucamine-dithiocarbamate (MGD)2-Fe2+ complex was used for the NO radical spin trapper in aqueous solution. The intensity of the ESR signal of the spin adduct formed by bubbling smoke from one cigarette increased gradually with time over 2 hours at about 20 degrees C and was constant for 2 days or longer. The time course of the production of the NO radical followed the rate equation y = 1520(1-e-0.018t) for the first-order reaction up to around 25 min after mixing of Fe2+ solution and then slowly approached the maximum value determined by the concentration of the spin adduct. These findings suggest that NO radical is produced slowly from NO radical donors such as amine .NO complexes, peroxinitrite (ONOO-), and other reactants such as nitrogen oxides (NOx), which are produced from the smoke of tobacco leaves, and suggest that its generation could be involved in the decomposition or cleavage of such substances.