The x-ray crystal structures of (I), the base 4030W92, 5-(2,3-dichlorophenyl)-2,4-diamino-6-fluoromethylpyrimidine, C11H9Cl2FN4, and (II) 227C89, the methanesulfonic acid salt of 5-(2,6-dichlorophenyl)-1-H-2,4-diamino-6-methylpyrimidine, C11H11Cl2N4·CH3O3S, were carried out at low temperatureA detailed comparison of the two structures is given.Structure (I) is noncentrosym., crystallizing in space group P21 with a 10.821(3), b 8.290(3), c 13.819(4) Å, β 105.980(6)°, Z = 4 (two mols. per asym. unit) and dc = 1.600 mg/m3.Structure (II) crystallizes in the triclinic space group with a 7.686(2), b 8.233(2), c 12.234(2) Å, α 78.379(4), β 87.195(4), γ 86.811(4)°, Z = 2, dc = 1.603 mg/m3.Final R indexes [I > 2σ(I)] are R1 = 0.0572, wR2 = 0.1003 for (I) and R1 = 0.0558, wR2 = 0.0982 for (II).R indexes (all data) are R1 = 0.0983, wR2 = 0.1116 for (I) and R1 = 0.1009, wR2 = 0.1117 for (II).Crystallog. data are given.5-Phenyl-2,4 diaminopyrimidine and 6-phenyl-1,2,4 triazine derivatives, which include lamotrigine (3,5-diamino-6-(2,3-dichlorophenyl)-1,2,4-triazine), were studied for some time for their effects on the central nervous system.The three dimensional structures reported here form part of a newly developed data base for the detailed study of members of this structural series and their biol. activities.