The activity of the enzyme serotonin N-acetyltransferase (NAT) in the rat pineal gland exhibits a large circadian rhythm, with peak activity occurring at night. This rhythm is dependent on stimulation of the pineal gland by neurons whose cell bodies are in the superior cervical ganglia and whose axons reach the gland via the internal carotid nerves (ICNs). Two days after both ICN were cut, crushed, or frozen, nighttime NAT activity was decreased by 90%. The remaining low level of enzyme activity was not affected by decentralization of the superior cervical ganglia. Thus, this enzyme activity did not depend on the activity of neurons in these ganglia. Bilaterally lesioning the ICN also abolished the neuronal uptake of norepinephrine in the pineal, further indicating that the sympathetic innervation of the gland had been destroyed. Three months after crushing both ICNs, nighttime NAT activity was only 20% of control values. However, in these animals, bilateral decentralization of the superior cervical ganglion reduced this low level of NAT activity by 90%. Thus, NAT activity, although low, was again dependent on sympathetic nerve stimulation. In contrast to this rather small recovery of nocturnal NAT activity, the norepinephrine uptake capacity of the gland recovered to 60% of control values. A similar discrepancy between the extent of recovery of NAT activity and of norepinephrine uptake was observed when the ICNs were frozen rather than crushed. To determine to what extent the sympathetic nerves that had reinnervated the pineal gland in these lesioned animals were capable of regulating NAT activity, their cervical sympathetic trunks were stimulated electrically at 5 Hz for 3 hr during the daytime.(ABSTRACT TRUNCATED AT 250 WORDS)