3-Deoxyl-1,2-O-isopropylidine-3-(3-methylureido)-alpha-D-ribofuranose (2) was converted to 1,2,5-tri-O-acetyl-3-deoxy-3-(3-acetyl-3-methylureido)-D-ribofuranose (4) and the corresponding glycosyl chloride (7). These sugars were converted to the 3-deoxy-3-(3-methylureido)-beta-D-ribofuranosyl derivatives of adenine (6c), 2-chloroadenine (6d), cytosine (6f), and uracil (6g). Nitrosation of these nucleosides gave the corresponding 3-methyl-3-nitrosoureidonucleosides 8c,d,f,g. 5'-Amino-5'-doxyadenosine (10a), 5'-amino-5'-deoxyuridine (10b), and 5'-amino-5'-deoxycytidine (10c) were all converted to the corresponding 5'-(methylureido)-5'-deoxynucleosides 15a--c by reaction with methyl isocyanate. Nitrosation of these compounds gave the methylnitrosoureidonucleosides 16a--c. These nitrosoureas, potential active-site-directed irreversible enzyme inhibitors, showed little cytotoxicity or activity against leukemia L1210 in vivo.