Anterior and posterior cortical lesions bilaterally produced at ages 18-20 days or at ages 74-76 days and unilateral lesions including both anterior and posterior cortical surfaces at early and later ages are shown to have different effects on the performance of rats reared in a complex environment and tested on a series of simple barrier tasks. Test scores showed no deterioration as a function of frontal lesions. For posterior lesions and for unilateral lesions the degree of test defect was correlated with the size of lesion; in both instances this appeared due to amount of encroachment on the posterior cortex. Less defect was observed in early than in late lesions in the posterior group; there were no differences attributable to time of operation in the other groups. 17 references. (PsycINFO Database Record (c) 2006 APA, all rights reserved)