Acute toxicity of pyridostigmine in rats: histological findings.

To assess the morphological changes due to pyridostigmine (Pyr), sublethal doses of Pyr (20 and 40 mg/kg body weight) were given to adult rats through a gastric tube. Within 24 h, acute focal necroses, leukocytic infiltrates and marked changes in the motor endplates appeared in the skeletal muscle. The changes were more evident in the diaphragm than in the quadriceps muscle. The acetylcholinesterase activity in the whole blood and in the erythrocytes was reduced to considerably less than one-half of the normal value. The histological changes induced by Pyr have not been reported to date, but are similar to those observed in experimental poisoning with other anticholinesterase agents such as carbamates and organophosphates.