A sural nerve and a muscle biopsy study of patients with chronic insecticides poisoning, with teased fiber preparations, routine pathologic studies of nerves and histochemistry of muscle is reported. The sural nerves of ten patients were studied and a teased fiber preparation was done in nine. The tenth patient had only fibrosis and no myelin was found. The sural nerves were abnormal in all patients and the teased fiber preparation resulted in preponderance of type C, D and large amount of G type fibers, according to Dyck's classification. These fibers had enlargement of the axon and myelin sheat, also seen in routine sections. The muscle biopsy with routine and histochemistry methods was done in 8 cases; in 6 there was found signs of denervation; the remaining cases were normal, but these were proximal muscles. Te authors conclude that the process primarly interfere with the functions of the axons, with distal axonal degeneration and a dying back phenomen.