113Cd-NMR studies have been used to elucidate the structure of the metal-binding sites in mammalian and invertebrate ( Scylla serrata) metallothioneins (MTs). Chemical shift data have shown that all Cd ions are tetrahedrally coordinated to four cysteine thiolate ligands with single cysteinyl sulfurs bridging adjacent metals. Homonuclear decoupling experiments have shown that the 7 g-atoms of metal bound per mole of mammalian protein are located in a three- and a four-metal cluster while the 6 g-atoms of metal in the invertebrate MT are located in two three-metal clusters. The different metal binding affinities of the two mammalian clusters have been determined by 113Cd-NMR. The three-metal cluster prefers Cu greater than Zn greater than Cd whereas exactly the reverse order applies in the four-metal cluster. Proteolytic cleavage of the protein produced a 32-residue fragment which contained the four-metal cluster and demonstrated the presence of two separate domains in the protein. 500 MHz 1H-NMR has been employed to elucidate the arrangement of these metal clusters in the tertiary structure of the protein. The 1H resonances were assigned from their scalar and dipolar connectivities obtained from extensive one and two-dimensional NMR experiments. A specific application of 2D correlation spectroscopy ( COSY ) to the assignment of the 1H resonances in crab MT-1 is discussed. A molecular model, representing the three-dimensional solution structure of this protein, has been constructed based on an analysis of all these data. Detailed structural features of this model are discussed, with particular emphasis on their relationship to the function and evolution of the protein.