Early histological studies carried out in the sciatic nerve of mice ch
ronically infected with Trypanosoma cruzi showed demyelination and sca
nty axonal degeneration. The experiments reported in this paper were d
esigned to assess the functional state of the sciatic nerve and of som
e of the muscles it supplies. For these purposes 14 mice were infected
with trypomastigotes (clon K-98, CA-I strain) 12 months before the in
vestigation. Results were compared with 13 normal mice matched by age
and weight. Hamstring muscles were studied electromyographycally by me
ans of a fine coaxial needle electrode and the sciatic nerve action po
tential characteristics were recorded with surface electrodes. All the
experiments were carried out in vivo. In the infected mice the electr
omyogram showed that some motor unit potentials has enlarged amplitude
and duration and increased number of phases, suggesting that the size
of their territories had been enlarged, probably through axonal colla
teral sproutings and reinnervation of muscle fibers previously relinqu
ished by their original innervation. The sciatic nerve action potentia
l of the infected animals showed diminished amplitude and prolonged la
tency. These features signal reduced number of functional axons within
the nerve and demyelination of the remaining conducting fibers. These
findings are in line with the histological evidences of the involveme
nt of the peripheral nervous system in Chagas disease and give additio
nal information about the functional state of the peripheral nerves in
the experimental model.