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1. The cholinergic agent carbachol (0·5 mg/kg) produces within 2 hr a two- to threefold increase in the activity of hepatic tyrosine transaminase in the adrenalectomized fasted rat.

2. The carbachol-evoked rise in enzyme activity is prevented by pretreatment with atropine.

3. Cycloheximide, an inhibitor of protein synthesis, prevents the carbachol-evoked rise of enzyme activity; actinomycin D, an inhibitor of RNA synthesis, partially reduces it.

4. Atropine or unilateral (right or left) vagotomy suppresses the daily rhythm of tyrosine transaminase by attenuating the normal evening peak but not the normal lower daily levels of the enzyme.

5. Electrical stimulation of the right vagus results in a significant elevation of tyrosine transaminase activity within 2 hr.

6. The observations suggest that activity of vagal cholinergic nerves produces an RNA-dependent induction of hepatic tyrosine transaminase through a process requiring cholinergic-receptor stimulation. Intermittent activity of this vagal cholinergic system may contribute to the regulation of the daily rhythm in this enzyme.