The effect of black and green tea on the NO system was tested. Beside a general effect i.e. NO scavenging, the teas interacted with good, i.e, relaxation of the aorta, and bad effects, i.e. peroxynitrite scavenging, of NO. Green tea was a better NO and peroxynitrite scavenger than black tea. Epigallocatechin gallate was the major identified contributor to both the peroxynitrite and NO scavenging. The theaflavins, only present in black tea, also had a substantial contribution to the NO scavenging of black tea. The teas were found to have only a minor and nonspecific effect on the NO mediated vasorelaxation. Based on these results it is concluded that tea discriminates between the good and bad effects of NO; tea is likely to prevent NO toxicity primarily.