The activity of ATP-citrate lyase in homogenates of five selected rat brain regions varied from 2.93 to 6.90 nmol/min/mg of protein in the following order: cerebellum less than hippocampus less than parietal cortex less than striatum less than medulla oblongata and that of the choline acetyltransferase from 0.15 to 2.08 nmol/min/mg of protein in cerebellum less than parietal cortex less than hippocampus = medulla oblongata less than striatum. No substantial differences were found in regional activities of lactate dehydrogenase, pyruvate dehydrogenase, citrate synthase or acetyl-CoA synthase. High values of relative specific activities for both choline acetyltransferase and ATP-citrate lyase were found in synaptosomal and synaptoplasmic fractions from regions with a high content of cholinergic nerve endings. There are significant correlations between these two enzyme activities in general cytocol (S3), synaptosomal (B) and synaptoplasmic (Bs) fractions from the different regions (r = 0.92-0.99). These data indicate that activity of ATP-citrate lyase in cholinergic neurons is several times higher than that present in glial and noncholinergic neuronal cells.