Post-transcriptional gene silencing (PTGS) is an important mechanism for regulation of plant gene expression and virus-plant interactions. To better understand this process, the heavy metal cadmium was identified as a specific inhibitor in two different PTGS systems, constitutive and inducible. The pattern of cadmium-induced inhibition of PTGS allowed several insights into PTGS development. First, cadmium treatment prevented only systemic but not local onset of PTGS, uncoupling between these two modes of PTGS. Second, non-toxic, but not toxic, levels of cadmium inhibited PTGS, suggesting induction of a pathway that interferes with PTGS. Third, cadmium effects on PTGS closely paralleled those on the movement of tobamoviruses, suggesting that both processes may share common steps in their systemic transport pathways. Interestingly, these effects of cadmium do not represent a general property of toxic metal ions because two other such elements, that is zinc and aluminum, did not interfere with PTGS and viral systemic movement.