An aversion to saccharin flavored water (.1%) was conditioned in male albino rats using X-rays as the noxious stimulus. The time interval between CS (saccharin) and US (X-ray) was varied for different groups and it was noted that an aversion could be formed when these stimuli were separated by as much as 12 h. Additional groups were run with 4% sucrose as the CS and the maximum effective time interval between CS and US was approximately 6 h.