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Sixty percent of the dopamine 0-hydroxylase (DBH) (EC 1.14.2.1) activity in rat brain was found to be bound to membrane fractions. The addition of Triton X-100 solubilized DBH completely and permitted the estimation of the total enzyme activity. In homogenates the DBH could be detected in the absence of Triton X-1 00 but was 30-45% of this total enzyme activity; in crude synaptosomal fractions this value was only 10%. Incubation of brain slices in media containing 2 X 1 0-3 M acetylcholine (ACh) led to an increase by a factor of 2-2.5 in the activity of DBH measured in the absence of detergents; however, total enzyme activity remained the same. The addition of 6 X 10-'M K+ in place of ACh produced a similar effect; however, the increase in the activity of DBH measured in the absence of detergents was only 25%. The effect of ACh could be reversed by removing it from the medium, or by the addition of diethyl p-nitrophenyl phosphate (paraoxon) 4 X 10W5M. The addition of eserine 4 X 10-4M increased the effect of ACh at 15 min of incubation. Neither acetate nor choline had any effect. Intact cell boundaries appeared to be necessary, since addition of ACh after complete homogenization caused no increase in activity. ACh caused a (9-lO)-fold increase in the specific activity of membranebound DBH but no change in the specific activity of the enzyme in cytosol. Although the effect of ACh mimicked the effect of Triton X-100, it was not due to the solubilization of DBH. The presence of ACh did not increase the release of DBH into the incubation medium.