Certain types of environmental stimulation administered during critical periods of neural development can enduringly modify adult behavior. The present experiments show that postnatal handling of Sprague-Dawley rats (administered from postnatal days 1 to 22) and/or living in an enriched environment (EE; from weaning until the age of 100 days) clearly improved the ability to learn a two-way active avoidance task in adulthood. In addition the results demonstrated that postnatal inconsistent stimulation (from postnatal days 1 to 22) impaired avoidance acquisition in the same task. This detrimental effect of inconsistent stimulation was reversed by EE. Our findings provide evidence that different types of early experience can influence learning abilities in distinct directions and with different strengths.