Podetia of the terricolous lichen Cladina stellaris (Opiz) Brodo, collected in an unpolluted rural area in N. Finland, were wetted with either H2O, diluted acidic solutions of H2SO4, HNO3 and NaHSO3, simulating acidic rain, or the following salts: K2SO4, KCl, CuSO4, CuCl2, Cu(NO3)3, ZnSO4, ZnCl2, Zn(NO3)2, FeSO4, FeCl2, Fe2(SO4)3, FeCl3 and Fe(NO3)3. The samples were further exposed to combined treatments in both acidic solutions, simulating acidic rain, and heavy metal salts in solution. Lichen samples wetted with H2O at pH 6·8, diluted solutions of H2SO4, HNO3 or a mixture of these two acids produced low concentrations of endogenous ethylene. The application of NaHSO3 greatly increased the production of ethylene. The application of KCl induced a higher rate of ethylene production than the application of K2SO4 solutions. The application of Cu-containing solutions enhanced the production of ethylene. The influence of Zn was smaller than that of Cu. Iron was the most effective heavy metal to promote the production of ethylene: very high ethylene concentrations were detected upon the application of FeCl2. Combined treatments in H2SO4 or H2SO4+HNO3 followed by either FeCl2 or FeSO4, yielded higher concentrations of ethylene than the same treatments in a reversed order. The role of Fe ions in the production of ethylene is discussed in reference to previous works dealing with ethylene production in higher plants, fungi and algae.