In adrenalectomized fasted rats transection of the spinal cord at C7-C8 or placement of bilateral electrolytic lesions in the lateral hypothalamus when performed in the morning interrupted the daily rhythm of hepatic tyrosine transaminase by elevating low (AM) enzyme activities to high (PM) levels; lesions placed in PM did not affect the late afternoon rise in enzyme activity.2. Bilateral thalamic lesions had no affect on enzyme activity.3. The activity of hepatic catechol-O-methyl transferase was unaffected by hypothalamic lesions.4. The lesion-evoked rise of tyrosine transaminase activity was abolished by exogenously administered norepinephrine.5. Cycloheximide blocked the rise of tyrosine transaminase activity caused by hypothalamic lesions.6. The results suggest that rhythmic activity of sympathetic nerves governed by lateral hypothalamus contribute to regulation of the daily rhythm in tyrosine transaminase by regulating the release of norepinephrine peripherally; norepinephrine may block the daily rise of enzyme by interfering with protein synthesis, possibly of new enzyme, by competing with pyridoxal co-factor.7. It is proposed that alternating activity of sympathetic-adrenergic and vagal-cholinergic nerves to liver, controlled by the C.N.S., contribute to rhythmic activity of hepatic tyrosine transaminase.
