The neurotoxic action of a synthetic pyrethroid, a deltamethrin formulation (Decis), was studied in adult rats. Adult male albino rats received deltamethrin in formulation at a dose of 7.0 mg/kg body weight/d in corn oil orally for 15 d. Deltamethrin-exposed rats exhibited a decrease in body weight from d 9 onward, which was significantly lowered at d 15 of exposure. Administration of deltamethrin markedly increased the wet weight of the hippocampus and pons medulla region without much affecting the weight of frontal cortex, corpus striatum, hypothalamus, and cerebellum. A significant increase in the activity of monoamine oxidase was observed in frontal cortex, hippocampus, and cerebellum, and acetylcholinesterase activity was markedly increased in frontal cortex, corpus striatum, hippocampus, cerebellum, and pons medulla. The activity of Na+,K(+)-ATPase showed marked decrease in frontal cortex, hippocampus, and cerebellum following deltamethrin exposure. The polyamine concentration in brain regions was significantly affected, and all three polyamines showed marked alterations in the cerebellum. Deltamethrin significantly increased the spontaneous locomotor activity and aggressive behavior. Maze learning was markedly decreased. Morphological changes in Purkinje neurons in the cerebellum were observed in deltamethrin-exposed rats. Results suggest significant neurochemical and neuromorphological changes, which may culminate in perturbed synaptic function following deltamethrin exposure in rats.