Serum melatonin concentration and its day-night difference in pinealectomized, stereotaxically grafted rats with pineal transplants was examined. The nighttime serum melatonin concentration increased significantly only in pinealectomized rats that received two pineal transplants. In neither pinealectomized rats receiving two pineal glands and one cotransplant of superior cervical sympathetic ganglion nor pinealectomized-ganglionectomized rats receiving two pineal transplants was there a demonstrable increased in serum melatonin concentration. Although some pineal transplanted hosts demonstrated increases in serum melatonin concentration, there was no day-night variation in serum melatonin concentration. This may be due to a number of mitigating factors. Pineal grafts may not receive the appropriate functional reinnervation from the host brain due to the location of the transplantation in the cerebral ventricle or due to lack of sufficient time for the growth of invading host neurites.