Adult male albino rats were orally infected with Trichinella spiralis (T. spiralis) larvae (400 larvae/rat). The cholinesterase (ChE) activity was then determined in the serum, brain, spinal cord, liver, stomach, intestine, heart, diaphragm and gastrocnemius muscle of the infected rats at different time intervals (15, 30, 45, 60 and 90 days) after infection. The enzyme activity was inhibited at all the time intervals in the brain, liver, heart and diaphragm while it increased progressively in the serum. In the spinal cord the ChE activity was inhibited at 15, 30 and 45 days postinfection but was markedly increased thereafter. In the intestine, stomach and gastrocnemius muscle the enzyme activity was generally increased. These alterations in ChE activity may be due to the stress effect due to the presence of the larvae in the tissues and/or the toxic effect or their metabolites.