The formation and structure of cobalt(II) and nickel(II) complexes of glycylhistamine (Gly-Hist) and sarcosylhistamine (Sar-Hist) have been studied by pH-metric and H-1-NMR methods. An EPR study of the analogous, diamagnetically diluted copper(II) complex is also included. In the pH range 3-9, monomeric 1:1 and 1:2, more or less deprotonated complexes are formed (MLH, ML, MLH(-1), MLH(-1)(OH), ML(2), ML(2)H(-1), M = Co(II), Ni(II)) depending on the pH and metal-to-ligand ratio. The interaction between ligands and nickel(II) at pH > 9 was characterized by the formation of imidazole-bridged (through both N-1 and N-3 nitrogens) polynuclear species (NiLH(-2))(n), with {4N} coordinated metal centers. In the case of Gly-Hist, the H-1-NMR study showed the existence of a single oligomeric (probably tetrameric, n = 4) species, while the steric hindrance of the N-CH3 group in Sar-Hist produces a series of coexisting oligomers, with n greater than or equal to 4.