3-Deoxy-1,2-O-isopropylidine-3-(3-methylureido)-.alpha.-D-ribofuranose was converted to 1,2,5-tri-O-acetyl-3-deoxy-3-(3-acetyl-3-methylureido)-D-ribofuranose and the corresponding glycosyl choloride. These sugars were converted to the 3-deoxy-3-(3-methylureido)-.beta.-D-ribofuranosyl derivatives of adenine, 2-chloroadenine, cytosine and uracil. Nitrosation of these nucleosides gave the corresponding 3-methyl-3-nitrosoureidonucleosides. 5''-Amino-5''-deoxyadenosine, 5''-amino-5''-deoxyuridine and 5''-amino-5''-deoxycytidine were all converted to the corresponding 5''-(methylureido)-5''-deoxynucleosides by reaction with methyl isocyanate. Nitrosation of these compounds gave the methylnitrosoureidonucleosides. These nitrosoureas, potential active-site-directed irreversible enzyme inhibitors, showed little cytotoxicity or activity against murine leukemia L1210 in vivo.