Activities of Acetylcholinesterase (AChE) in brain and erythrocytes, pseudocholinesterase (PChE) in plasma and non-specific carboxylesterase (NSE) in brain, liver, plasma and kidney were assayed at weekly intervals in growing male rats fed dietary pirimiphos-methyl at dosages of 1000, and 1500 ppm for a period of 28 days. Significant inhibition of tissue esterases were observed at the end of 7 days at both the dosages and the enzyme activities remained markedly inhibited at subsequent intervals. Activities of all enzymes (excepting Brain AChE) returned to normal levels following a 7-day maintenance on the insecticide-free diet implicating that pirimiphos-methyl induced little or no permanent tissue damage. The results suggest that plasma NSE level may also serve as a sensitive indicator in monitoring OPI exposure.