Abstract Interhemispheric synthesis of behavior from lateralized engrams established during unilateral cortical spreading depression (CSD) was studied in rats. In the competitive synthesis experiments animals learned with the left and right hemisphere to avoid the left and right side of an apparatus respectively. With the brain intact the rats either preferred one of the above avoidance reactions (70%) or stayed in the middle part of the apparatus (30%). The last reaction corresponded to a subtraction of the two opposing avoidance gradients. In the cooperative synthesis experiments rats learned two simple avoidance reactions, one of which could be simplified considerably by using the information acquired during the second task. Synthesis of this type was more efficient in rats in which the partial reactions were independently stored in each hemisphere, than in animals which learned them with the whole brain. Some of the differences between the interhemispheric and intrahemispheric synthesis may be due to the fact that the usual sequential acquisition of memory traces in the normal brain is replaced by parallel spatial storage under conditions of lateralized learning.