Neuropeptide Y (NPY)-like immunoreactivity (LI) in the iris-ciliary body of rats kept under constant darkness (DD) and in 12 h light-12 h dark cycle (LD) was determined by enzyme immunoassay. NPY-LI contents in the iris-ciliary body were found to oscillate in circadian fashion under DD and LD conditions, with a peak at about circadian time 12 (CT 12) and a trough at around CT 0. Unilateral superior cervical ganglionectomy caused a significant decrease in NPY-LI levels in the sympathectomized eye compare to the contralateral intact eye, independent of lighting phase. These results suggest the presence of an endogenous circadian rhythm in NPY-LI content in the rat iris-ciliary body, and the possible involvement of a sympathetic input from the superior cervical ganglion.