In experimental protocols with humans and non-human primates, cerebrospinal fluid (CSF) concentrations of 3-methoxy-4-hydroxyphenylethylene glycol (MHPG), the predominant end-product of norepinephrine metabolism in the mammalian central nervous system (CNS), have been widely used as an index of the rate of CNS norepinephrine metabolism. However, an earlier investigation showed that there was slow but free exchange between plasma and CSF MHPG. To define more precisely the time-course of equilibration of plasma and CSF MHPG, we intravenously administered 100 micrograms/kg of [2H3]-MHPG to drug-naive squirrel monkeys. Measurements were made of the concentrations of [2H3]- and [1H]-MHPG in plasma and cervical CSF samples collected at time points from 10 min to 4 hr thereafter. The results indicated that neither plasma nor CSF concentrations of [1H]-MHPG changed during the course of the experiment, and that [2H3]-MHPG appeared in the CSF within 10 min of intravenous administration. The maximal plasma and CSF concentrations of [2H3]-MHPG were 7.6- and 2.3-fold higher than the respective concentrations of [1H]-MHPG. The plasma and CSF pools of [2H3]-MHPG reached concentration equilibrium within 30 min, and thereafter the temporal decline in concentration of [2H3]-MHPG was the same in plasma and CSF. These results demonstrate that MHPG rapidly crosses from plasma to CSF, and support the suggestion that this factor be included in any attempts to estimate norepinephrine turnover in the CNS from measurements of steady-state MHPG concentrations in CSF or plasma.