Like all other animal species terrestrial pulmonate snails require Cu as an essential trace element. Specifically, snails need Cu as a basic constituent of hemocyanin, their respiratory protein. On the other hand, Cu sulfate can be used as a molluscicide against certain snail species considered to be pest organisms. The homeostatic regulation of Cu must therefore be a pivotal goal of terrestrial pulmonates in order to survive under both physiological and supra-physiological conditions of metal exposure. Upon administration of Cu, snails accumulate the metal nearly equally in most organs, with highest concentrations in the mantle, foot and midgut gland. Quantitative studies in connection with high performance liquid chromatography and electrospray ionization mass spectrometry reveal that a certain fraction of Cu in the snails is bound to a highly oxidation-susceptible Cu-metallothionein (Cu-MT) isoform which occurs in most organs at constant concentrations, irrespective of whether the animals had been exposed to physiological or elevated amounts of Cu. In-situ hybridization demonstrates that at the cellular level, the Cu-binding MT isoform is exclusively expressed in the so-called pore cells (or rhogocytes) which can be found in all major snail organs. The number of pore cells with Cu-MT mRNA reaction products remains unaffected by Cu exposure. Rhogocytes are also major storage sites of Cu in a granular form, the metal quickly entering the snail tissues upon elevated exposure. The number of rhogocytes with granular Cu precipitations strongly increases upon Cu administration via food. Thus, whereas Cu-MT in the rhogocytes represents a stable pool of monovalent Cu⁺ which apparently serves physiological tasks and is not influenced by metal exposure, the granular Cu precipitations form a second, quickly inducible and more easily available pool of the metal which serves Cu regulation by responding to supra-physiological metal exposure.