Between birth and 21 days of age, 38 rats were raised in maternity cages or in enriched free environments. At 21-50 days of age, 1/2 of each group were in laboratory cages and 1/2 were in free environments. Then all Ss were kept in laboratory cages until Hebb-Williams problem-solving tests began at an average age of 371 days. Both preweaning and postweaning enriched experiences independently improved problem-solving performance. The findings support Hebb's hypothesis that enriched experiences in early life result in permanent brain changes. (PsycINFO Database Record (c) 2006 APA, all rights reserved)