Oral administration of 2,4-dichlorophenoxyacetic butyl ester (2,4-Dbe) at a dose of 69 mg/kg/day to nulliparous females had no deleterious effects on either open field (OF) and rotarod performance. By contrast, dams treated with 2,4-Dbe during pregnancy exhibited impairments of OF activity, rotarod performance and improved active avoidance learning (AAL) retention. Administration of 2,4-Dbe to 90-day-old intact male rats depressed spontaneous OF activity, acquisition of conditioned avoidance responses (CARs) and rotarod endurance, but improved AAL performance. Castration itself impaired performance in the rotarod test, and improved AAL, but did not alter OF activity significantly. The effects of castration were reversed by exogenous testosterone. In gonadectomized rats, 2,4-Dbe prevented the reversal of the effect of testosterone on the influence of castration on behavior if given concomitantly with the testosterone. However, when the 2,4-Dbe treatment started seven days after testosterone, the 2,4-Dbe effects on OF, rotarod and AAL behaviors were reinstated. Thus, testosterone appears to be important for causing the toxic effects of 2,4-Dbe in rats.