The reaction of 3-azolyl-6-R-1,2,4,5-tetrazines with guanidine afforded a series of new 3-guanidino-6-R-1,2,4,5-tetrazines. When treated with hydrogen peroxide, these compounds can undergo oxidative transformation of the tetrazine ring to form 2,5-disubstituted 1,3,4-oxadiazoles. The reaction of the new tetrazines with concentrated nitric acid leads to the nitration of functional groups in the tetrazine ring.