The effect of a lesion of the dorsal septum on active and passive type of avoidance reactions of adult male Wistar (W) and Long-Evans (LE) rats was studied. The rate of acquisition and extinction of the reaction was studied by 3 different testing methods. The animals were operated on when juvenile (30 days) or adult (90 days). The experiments were always started 50 days after the operation, when the "rage syndrome" was no longer present. In the three different tests we found a single common variable--the strain factor. It was this that determined whether the operation was effective, as well as the direction of deviations and the operation age which led to manifest changes. In W males the lesion did not impair either the acquisition or the extinction of an active avoidance reaction (AAR); the passive avoidance reaction (PAR) was acquired and extinguished more slowly after a lesion in adulthood. The spontaneous passive avoidance reaction (i.e. preference of a small, dark space) was likewise negatively affected by the operation. In intact LE animals the AAR was extinguished more slowly than in intact W males; after a lesion produced in juvenile or adult age extinction was speeded up, so that there were no differences compared either with intact or with septal W animals. Intact LE rats also acquired a PAR more slowly than W rats; a septal lesion led to faster acquisition, irrespective of the age at which the operation was performed, so that the rate for septal LE rats were the same as for intact W animals. The extinction of this reaction took longer after an operation at juvenile age and the rate for these septal LE rats were the same as for those of septal W individuals operated on at 90 days. The spontaneous PAR was qualitatively poorer in intact LE animals than in intact W rats, since only 40% of them preferred the small, dark space. A lesion was followed by improvement, which was especially marked after operation on the 30th day, when all the animals preferred this space, and in a very short time, so that they were equal to intact W males. Correlation of the acquisition and the extinction rate showed that there was imbalance of excitation and inhibition processes in the AAR of the W controls, that they were balanced in the PAR and that the lesion reversed these relationships. Both processes were balanced in the LE control and a septal lesion did not alter the situation.