Factors involved in the metal homeostasis (metallothioneins, phytochelatins, copper-chaperons) in barley (Hordeum vulgare) were identified and their expression characterized. By this, two different situations when changes in metal homeostasis occur were analyzed: firstly, analyses of senescening primary leaves, when due to cellular decomposition (especially of the chloroplast), formerly bound metals are released from their metal containing proteins and to some extent transported to other parts of the plant. Secondly, barley seedlings exposed to toxic amounts of either essential or non-essential metals and effects on the primary leaves were investigated. The comparison of the plant response to both situations reveals that both the physiological and the molecular level show great similarities. This work revealed that barley possesses at least nine different metallothionein genes and the mRNA of six of these genes were identified as cDNAs for the first time. Gene expression analyses showed that the different metallothioneins are expressed differently with respect to their tissue specificity, their developmental regulation, and their response to heavy metal stress. This indicates different functions of the various metallothioneins in the detoxification of metals, respectively, in the maintenance of the metal homeostasis. However, HPLC analyses show phytochelatin (PC2 and PC3) accumulation in leaves only following a long term exposure of plants to cadmium, but not to copper or during leaf senescence, suggesting that PC are neither involved in the senescence program nor in the copper detoxification in leaves. The analyses of four novel cadmium induced genes (HvBCB, Cdi1,Cdi2, Bsi1) showed that their expression does not depend only on the non-essential, non-redoxactive metal cadmium but can be triggered by high concentrations of the essential, redox-active metal copper and during late stages of leaf senescence.