Using a conditioned suppression paradigm, differential conditioning between short (0.7s) and long (4.9s) US was studied in three groups of rats where the US intensity was varied across groups. Group Weak was trained with the intensity of 60V (0.21mA), while Group Medium was with of 120V (0. 43mA). Group Strong was trained with the intensity of 180V (0.64mA). All groups were assessed their conditioned suppressions to both CSs in probe sessions where no US was presented. The degree of differentiation increased in propotion with US intensity ; the greatest differentiation was obtained in Group Stong which was conditioned with the strongest shock. Group Weak did not show a significant differentiation. These results suggest that the US intensity determines the net difference of the US durations in the differential conditioning.