Wistar female rats were exposed to relatively mild concentrations of carbon monoxide (75 and 150 ppm) from day 0 to day 20 of pregnancy. The results show that prenatal exposure to CO (150 ppm) significantly impairs the acquisition of a two-way active avoidance task in 3-month-old male rats as well as the acquisition and reacquisition of this schedule in 18-month-old animals subjected to six daily 20-trial sessions. These deficits do not seem to be attributable to alterations of a non-associative nature, as the intertrial activity and the escape response latencies in CO exposed animals were not significantly affected with respect to controls. These findings, showing that gestational exposure to CO induces in rat offspring permanent learning and memory impairment, confirm that the offspring of smoking mothers may be at considerably greater risk than current epidemiological studies on birthweight and neonatal mortality suggest.