An existing method for measuring acetylcholine (ACh) and choline (Ch) is shown to be useful for measuring the turnover rate of ACh in mouse brain. Methyl-[3H]Ch is injected into mice. They are killed at different times by microwave irradiation and Ch and ACh extracted and separated by reverse-phase HPLC. Ch and ACh are converted to hydrogen peroxide by a post-column enzyme reaction. Hydrogen peroxide, which is directly related to the tissue content of Ch or ACh, is determined electrochemically. The fractions that correspond to the detector response for Ch and ACh are collected for the measurement of radioactivity. In this way specific radioactivities of endogenous Ch and ACh are estimated in the same sample. We used the specific radioactivity values determined by this procedure to estimate the turnover of ACh for striatum, cerebral cortex, and hippocampus of the mouse.