Functional decortication induced by cortical spreading depression (CSD) was used to estimate the significance of cerebral cortex for swimming and depth avoidance in arts. In the swimming test (5 gr sinkers, 36 degrees C water) the median swimming time was reduced from greater 120 minutes in the controls to 16 minutes in the bilaterally depressed rats. Depth avoidance in the physical and visual cliff situation (6 cm to the shallow and 48 cm to the deep surface) was unimpaired by unilateral CSF but was abolished by bilateral CSD. Combination of monocular occlusion with ipsilateral CSD deteriorated the visual cliff test but not the physical cliff behaviour. Functional decortication increased descent latencies and decreased the explaration rate in both tests. It is concluded that cerebral cortex plays an important role in the regulation of unlearned, innate activities with the overall behaviour of the organism.