The relationship between CSF somatostatin and peripheral thyroid hormones was assessed in 11 affectively ill patients before and during carbamazepine treatment. A direct relationship between CSF somatostatin and plasma thyroxine and free thyroxine, but not triiodothyronine, was observed both while patients were medication-free and during carbamazepine treatment. These first data in man are consistent with those in animals, suggesting a close interrelationship between somatostatin and thyroid regulation.