KINETICS OF PLASMA CHOLINE IN RELATION TO TURNOVER OF BRAIN CHOLINE AND FORMATION OF ACETYLCHOLINE1

—²H₄]Ch (2 μmol kg^-1 min^-1) was infused into both anaesthetized and conscious rats to study the kinetics of plasma and brain choline (Ch) and brain acetylcholine (ACh). A larger amount of endogenous Ch was found to leave the brain than enter, even in conscious animals. ²H₄]Ch was taken up into the brain where a portion was converted to ²H₄]ACh. Upon stopping the infusion, however, more ²H₄]Ch was found to leave than enter, indicating a source capable of generating Ch in brain which is labelled by infusion for 32 min. There appears, however, to be more than one source of Ch in the brain since the post mortem increase is not labelled following prolonged infusion. Thus, the brain Ch pool appears to be continually diluted by the sources within the brain to the extent of 93 per cent. During the infusion of ²H₄]Ch, the total levels of brain Ch and ACh did not increase. The brain Ch and ACh specific activities rose exponentially and appear to approach an asymptote at about 4 h. The source or sources of Ch within the brain produce Ch at a rate of 26·3 nmol g^-1 min^-1. The turnover of free Ch in the rat brain is 28·4 nmol g^-1 min^-1.