The pineal organ is a derivative and integral component of the brain. In phylogeny it has changed from a ‘third eye’ capable of direct achromatic and/or chromatic responses to light to an endocrine gland influenced by visual stimuli from the retina. The outer segment of pineal photoreceptors contains a microspectrophotometncally detectable photopigment and an opsin-immunoreactive protein. Certain pinealocytes endowed with an outer segment display fluorophores of 5-HT/HTP and can be regarded as photoneuroendocrine cells. These photoneuroendocrine cells resemble cerebrospinal fluid-contacting neurons. The intrinsic circuitry (wiring diagram) of pineal photoreceptor organs has been only partly deciphered; this holds true also for the neurotransmitters and synaptic patterns. Pineal photoreceptor organs are connected to the brain, i.e., to habenular, pretectal, thalamic, tegmental (reticular), and hypothalamic centers. The special anatomical and physiological features of the pineal organ enable it to translate photic information into neural and neuroendocrine responses.