Abstract: A method of producing 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) by nitrosation of 1,3-bis(2-chloroethyl)-urea utilizing as a nitrosating agent dinitrogen trioxide (N.sub.2 O.sub.3) in molar excess of theoretical wherein the molar excess of N.sub.2 O.sub.3 ranges from 10-200% and preferably from 10-20%. The nitrosation reaction is conventionally carried out in the cold from 0.degree. C to -20.degree. C and a non-aqueous solvent is utilized. The preferred non-aqueous solvent is of the chlorinated variety; i.e., methylene dichloride. Other preferred solvents include related halogenated compounds such as ethylene dichloride, nitro-compounds such as nitromethane, acetonitrile, and simple ethers such as ethyl ether. Other less preferred but operable compounds include esters such as ethyl acetate, simple ketones such as acetones, and chloroform.