Equivalence reactions were studied in 14 normal rats, 15 rats which had received cortical injuries subsequent to being trained on the black-white cards, and 15 rats which received cortical injuries preceding training. After being trained to react positively to a black circle on a white background which was juxtaposed with a white circle on a black background. After training was completed tests were made on 13 different critical pairs of stimuli. Cerebral injuries tended to increase the number of equivalent stimuli. For those rats trained previous to the operation there was a correlation of .746 [plus or minus] .084 between the size of the lesion and the increase in the number of equivalent stimuli after operation. A discussion is given of the theoretical implications of the findings.