We have previously reported that clock mutant mice on Jcl:ICR background show about 2-h delayed circadian profiles in body temperature, spontaneous activity and sleep-wake rhythm, and that they appear to be useful as a model of evening-type of individual. Hippocampal acetylcholine (ACh) release which is positively correlated with attention, learning and memory shows a circadian variation. In this study, changes in hippocampal ACh release in transitional phase from light (rest) to dark (active) period in clock mutant mice were monitored using an in vivo microdialysis method. Compared with wild mice, the increase in hippocampal ACh in the first 2 h of the active period in the mutant mice was suppressed in parallel with peak frequency in electroencephalogram theta rhythm. The molecular basis of the circadian system appears to have a strong effect on hippocampal cholinergic function, and is probably associating with individual temporal differences in voluntary behavior, cognition, learning and/or memory performance.