The rat pineal gland receives sympathetic innervation, via the right and left internal carotid nerves, from neurons whose cell bodies are located in the two superior cervical ganglia. Using fluorescence microscopy, we have examined the density and distribution of fluorescent profiles in the pineal gland after lesioning of the internal carotid nerves. Thirty-two hours after sectioning both internal carotid nerves, the density of the fluorescent profiles was 3% of that seen in sham-operated controls, indicating that the lesioned fibers had degenerated. Thirty-two hours after sectioning one internal carotid nerve, the density of the processes was decreased to approximately 50% of the control value. The magnitude of the decrease following a unilateral lesion was similar on both the right and left halves of the gland. Thus, the results suggest that each internal carotid nerve innervates both sides of the pineal gland. The implications of this overlap in the innervation from the two internal carotid nerves for recovery of pineal function after a unilateral lesion are discussed. Sections of pineal glands were also analyzed at later time points after a unilateral lesion. Two weeks after cutting one internal carotid nerve, the density of the fluorescent profiles had increased to greater than 80% of the control value. When the contralateral internal carotid nerve was cut 2 weeks after a unilateral lesion and the pineal gland was examined 32 h later, the density of the fluorescent profiles had decreased to 2% of the sham value. This suggests that all of the compensatory increase in adrenergic processes that takes place following the unilateral lesion is due to sprouting of the contralateral internal carotid nerve rather than to regeneration of the lesioned internal carotid nerve or to sprouting and ingrowth of other adrenergic neurons. It remains to be determined what, if any, functional significance this sprouting has, since the neurally dependent circadian rhythm in serotonin N-acetyltransferase activity in the pineal gland is restored to normal within 32 h after a unilateral lesion; that is, before significant sprouting has occurred.