Seventy-two rats were given 0, 5, 10, or 15 small rewarded trials prior to being shifted to large reward for the remainder of 45 trials. Each of the groups which experienced an upward shift in reward magnitude rapidly approximated the performance of a consistently large rewarded control group, and in no case was a positive contrast effect observed. The performance curves for all the groups remained interlaced for a rather protracted period of asymptotic training. The results were discussed in terms of their relevance to the “;ceiling effect”; hypothesis of the absence of positive contrast.