Two experiments investigated the importance of visual sensory modality in mediating enriched environmental effects during the preweaning period, and the importance of onset and duration of the enriched experience during the same period. Rat mothers and pups were exposed together in an enriched environment for either 7 or 11 days at different periods from Day 1 to Day 21. Also included was a group of rats which received only handling from Day 11 to Day 21 postpartum and another group in which the mothers were exposed to the enriched environment during the last trimester of pregnancy. Some rats were tested at 27 days of age, and others were tested at 64 days of age in the Hebb-Williams maze test. It was found that exposure in the enriched environment for a period of 7 days before the eyes opened improved rats' problem-solving behavior above that of control rats and to the level of rats which received such experience after the eyes had opened. Handling of rats did not improve problem-solving behavior, nor did exposure during the gestation period. It was concluded that vision is not the most important factor in mediating effects during the preweaning period, since improvement in problem-solving behavior can occur before the visual system is fully developed. It was suggested that the beneficial enrichment effects might be mediated by the mother, who, in some way, transmits additional stimulation to the infant rat during this early stage of development.