SUMMARY -Thirty five patients with acute Chagas' disease who demonstrated parasitaemia at the time of the investigation were submitted to a detailed electromyographical study. With their muscles at rest, 12 patients showed fibrillation potentials and/or positive sharp waves.On volitional contraction, 7 had short duration motor unit potentials (MUPs) and low polyphasic MUPs. On motor and sensory nerve fibers conduction studies, 20 disclosed values below the lower control limit within one or more nerves. Finally, 12 patients produced a muscle, decremental response on nerve supramaximal repetitive stimulation.The findings signal that primary muscle involvement, neuropathy and impairement of the neuromuscular transmission, either isolated or combined, may be found in the acute stage of human Chagas' disease. Compromiso de la unidad motora en la fase aguda de la enfermedad de Chagas humana.RESÚMEN -Treinta y cinco pacientes con el diagnóstico de enfermedad de Chagas en su etapa aguda, todos con parasitemia positiva en el momento de la investigación, fueron sometidos a estudio electromiográfico por técnicas convencionales. En reposo, 12 de ellos mostraron fibrilaciones y/o potenciales positivos. Durante la contracción voluntaria, en 7 pacientes los potenciales de unidad motora eran bifásicos de corta duración y polifásicos de baja amplitud. En 20 se encontró disminución de la velocidad de conducción motora y/o sensitiva en uno o mas de los nervios explorados. Finalmente, 12 pacientes mostraron caida de la amplitud del potencial muscular evocado por estímulo nervioso repetitivo supramáximo. Los hallazgos hechos señalan que durante la fase aguda de la enfermedad de, Chagas en el hombre puede producirse alteración primaria del músculo, neuropatía y compromiso de la transmisión neuromuscular, en forma aislada o combinadas entre si.In the past years, the involvement of the peripheral nervous system has been extensively reported in chronic human Chagas' disease 9-u and, more recently, the same type of lesion could be demonstrated in chronic experimental models 4,8.However, only few attempts have been done looking for the eventual damage of the peripheral nervous system in the acute stage of the human disease 2 .Some experimental data obtained from the mouse support the possibility of a very early injury of the peripheral nerve 5, besides the well-known myositis 6, shortly after the inoculation of the animal.The aim of the present study, has been to make a first approach searching for the involvement of the peripheral nervous system in patients affected by acute Chagas' disease who live in an endemic area. A previous and limited account of these findings has been published elsewhere 1.