Circadian rhythms are controlled by endogenous oscillators or clocks. These clocks exhibit a persistent period of approximately 24 h in constant conditions, a specific phase relationship to a periodic cue (zeitgeber) in the external environment, and plasticity in that the phase of the clock may be altered in response to a phase change in the zeitgeber. Although many processes exhibit circadian rhythmicity, the nature and location of endogenous clocks remain poorly defined. Recent evidence in vertebrates suggests that the mammalian suprachiasmatic nucleus and the avian pineal gland contain clocks that affect the rhythmicity of indoleamine metabolism. The vertebrate retina also exhibits a circadian rhythm of serotonin N-acetytransferase activity (NAT, EC 2.1.1.4), a key enzyme controlling melatonin synthesis, and of photoreceptor disk shedding. The latter process may be regulated by melatonin, and the immediate cellular events seem to be controlled locally within the eye. Although sustained oscillation and entrainment were not demonstrated, data suggesting that an ocular circadian clock influences disk shedding have been reported. We sought evidence for an ocular clock by studying retinal NAT activity in Xenopus eye cups maintained in culture and report here both sustained oscillation and entrainment of the in vitro system. The data indicate that in addition to the suprachiasmatic nucleus and pineal gland, the eye itself must be regarded as the locus of a circadian clock in vertebrates.