A histopathological study, with a light microscope, of experimental neuromyopathy produced by thallotoxicosis was undertaken in 40 newborn Wistar rats. Treatment consisted of a single i.p. injection of an aqueous solution of 16 mg kg-1 thallium(I) acetate 1 day after birth. Groups of ten animals were euthanized at either 8 or 50 days of age. Sural nerves, as well as peroneus muscle, were fixed in 10% formaldehyde solution for 15 days and then prepared for histopathological observation. At 8 days of age sural nerves of thallium-treated animals showed a moderate reduction in the large- and medium-sized fibres and several of the myelin sheaths had initial degeneration along the course of the axon. Interstitial oedema was found in both neural and muscular tissues. Distinct features of focal necrosis as well as small haemorrhages were seen in peroneus muscle. At 50 days of age, the lesions were more diffuse. Large and small myelinated fibres were found to be sinuous, fragmented and scanty. Alterations in the large- and medium-sized axons were seen and the myelin sheaths were altered along the course of the axon, suggesting a progressive distal axonopathy. Additionally, muscle fibres had myopathic changes with abnormal central nucleoli and the striated transverse fibres had disappeared in many areas of the sample. Several interstitial foci of muscular necrosis accompanied by phagocytosis and fibrosis were also present.