The activities of tryptophan 2,3‐dioxygenase (EC 1.13.11.11), indoleamine 2,3‐dioxygenase (EC 1.13.11.17), kynurenine 3‐hydroxylase (EC 1.14.13.9), kynureninase (EC 3.7.1.3), kynurenine transaminases, and pyridoxal phosphokinase (EC 2.7.1.35) in the liver, kidney and lung of rats were measured after administration of a single dose and repeated doses of dimethoate, carbaryl and fenvalerate, respectively. Ten percent LD50 of each insecticide was orally administered to a rat for a single dose, while 5% LD50 was orally given for five consecutive days as repeated doses. The control animals received the same volume of vehicle (polyethylene glycol 300). Body weight and organs weight losses were recognized only after repeated doses of dimethoate, while protein content remained constant compared to control animals. Repeated administration of dimethoate caused significant decrease in the activity of kynurenine 3‐hydroxylase (28.3% decrease in liver, and 32.5% in kidney), kynurenine 2‐oxoglutarate transaminase (EC 2.6.1.7) (40% in liver, and 24.2% in kidney), kynurenine‐ pyruvate transaminase (EC 2.6.1.‐) (24.5% in kidney) and pyridoxal phosphokinase (36.1% in liver). Repeated doses of carbaryl resulted in a significant decrease in the activity of apo‐tryptophan 2,3‐dioxygenase (42.8%), kynurenine‐2‐oxoglutarate transaminase (40% in liver), kynurenine‐pyruvate transaminase (30.6% in liver), and serine‐glyoxylate transaminase (EC 2.6.1.51) (47.9% in liver). Externally added insecticides at different concentrations to the incubation mixture resulted in an inhibition to tryptophan 2,3‐dioxygenase, while the other enzymes examined showed no change in their activities.