The latencies and incidence of deltamethrin-induced motor symptoms were compared in rats pretreated with saline or 3-acetylpyridine (3-AP) (75 mg/kg i.p. 4 days before). 3-AP produced considerable degeneration of the inferior olive and hence the climbing fiber afferents to the cerebellar cortex. The times of onset of whole body tremor and spontaneous choreiform episodes were accelerated significantly in lesioned rats. Therefore the olivocerebellar tract is not important in mediating deltamethrin symptoms, but is necessary to compensate for the disruption of motor activity and coordination induced by deltamethrin at another site. Cerebellar cyclic guanosine monophosphate (cyclic GMP) levels were elevated significantly during the deltamethrin syndrome. The increase in cyclic GMP levels correlated directly with the duration of motor symptoms. This suggests that deltamethrin has an indirect effect on cerebellar cyclic GMP levels. Lesion of the olive-cerebellar tract did not abolish this rise in cerebellar cyclic GMP levels. The sequential development of the motor symptoms, whole body tremor followed by choreiform episodes was found to have an additive effect in raising cerebellar cyclic GMP levels. This suggests that both the nature and the general level of motor activity can influence cerebellar cyclic GMP levels.