In both 2-amino-6-methoxy-4-(4-methylanilino)-5-nitrosopyrimidine, C 12 H 13 N 5 O 2 , (I), and ethyl N -[4-(1-adamantylamino)-2-amino-5-nitrosopyrimidin-6-yl]-3-aminopropionate, C 19 H 28 N 6 O 3 , (II), the nitrosopyrimidine unit is planar and the bond distances provide evidence for significant polarization of the electronic structures. In (II), the ethoxycarbonyl fragment of the molecule is disordered over two sets of sites with occupancies of 0.910 (4) and 0.090 (4). In the molecules of both compounds, there is an intramolecular N—H...O hydrogen bond. The molecules of (I) are linked into a chain of rings by a combination of N—H...O and C—H...O hydrogen bonds, while the molecules of (II) are linked by a two-centre N—H...N hydrogen bond and a three-centre N—H...(N,O) hydrogen bond to form sheets containing four distinct types of ring.