The behavior of 25 rats trained in a homogeneous shuttle box to escape unsignalled grid-shock was analyzed. Three categories of escape were distinguished: (1) species-specific fly away from the charged grid, (2) long-latency crossing preceded and accompanied by other behaviors that compete with the escape response, and (3) short-latency escape which followed an anticipatory postural pose. The animals displayed species-specific fly away only during the initial trials of a session. Subsequently long-latency crossings develops, reflecting a resistance to enter the opposite compartment. A measure based on a comparison of escape latency distributions in the two halves of the 1st session discriminates between good and poor learners. Subgroups of good and poor learners differed in performance efficiency in all five training sessions. Good learners were able to overcome the resistance to enter the opposite compartment and recall the learned short-latency escape.