Experiments on the effect of Cd, Cu, Ni, Pb and Zn, supplied as single salt solutions between 10 μM and 0·1 M, on the moss Rhytidiadelphus squarrosus showed little alteration to respiratory rates but reduced photosynthetic rates, and some membrane damage as assessed by K leakage. Two distinct morphological forms of the moss showed different responses to supplied heavy metal. Storage of material, after 30 mins exposure to heavy metals, resulted in a further decrease in the photosynthetic rate. Expressing the photosynthetic decline relative to the total heavy metal recovered from the moss showed a similar pattern. Transfer of metal from extracellular exchange sites into the protoplast was also demonstrated with storage after exposure. An approximately linear relationship was demonstrated between photosynthetic decline and intracellular heavy-metal concentration, irrespective of the duration of exposure or morphological nature of the material used. Photosynthetic decline on storage is concluded to be a response to additional metal stress rather than a progressive deterioration of the physiological process.