The structure of mouse skin collagen fibrils, after treatment with lithium chloride at a dose of 0.7 meq/kg of body weight, was studied by electron microscopy. Animals were sacrificed 1-day, 1-, 2- and 6-months after the end of 30 consecutive days experimental period. Fibrils in disarray interspersed with normal ones were seen, although there were areas where the normal parallel packing of fibrils was completely replaced by a random arrangement. In some regions this packing was interrupted by fibrils forming a 'hairpin loop' and in some others helical twisting was apparent. Lithium treated collagen fibrils had a marked decrease in mean diameter compared to normal with a high variability in width. Clusters of abnormal fibrils were found when viewed in cross-sections, constituted approximately 10% of the whole population with an irregular outline. A shorter experimental period (7 days) leads to the same features in collagen disorganization as with the longer experimental period (30 periods). However, in short lithium treatment, there is no high variability in fibril width and although fibrils are smaller than the control, the decrease in mean diameter is not so pronounced as with longer treatment. In general, fibril diameter becomes altered progressively with experimental time. Lower dose, 0.3 meq Li/kg, in short treatment (7 days), leads to a normal collagen organization although in some areas fibril twisting is observed. Fibril diameter is not considerably affected.