Using the fluorescence histochemical technique, yellow autofluorescent granules were observed in neurones of the arcuate and ventromedial hypothalamic nuclei of the rat (type I cells). In the same nuclei, neurones could be demonstrated showing a formaldehyde-induced yellow fluorescence (type II cells). Microelectrophoresis and special staining methods applied to the pineal gland revealed the autofluorescent compound to be a protein containing a relatively high content of tryptophan. It is probable that the formaldehyde-induced yellow fluorescence is due to the presence of serotonin. In view of investigating a possible functional relationship between the pineal gland and the parvocellular hypothalamic nuclei mentioned, hypothalamic type I and type II cells, as well as autofluorescent and serotonin-containing pinealocytes, if present, were quantified under the following experimental conditions: (1) p-chlorophenylalanine (pCPA) administration, (2) castration, (3) pinealectomy, and (4) pinealectomy followed by substitution using rat and sheep pineal extract. Administration of pCPA caused a decrease in the number of type II and an increase in the number of type I cells, both in the pineal gland and the hypothalamic nuclei. Castration, in contrast, was followed by an increase in the number of autofluorescent pinealocytes, but a decrease of autofluorescent neurones in the hypothalamic nuclei (type I cells) while the number of serotonin-containing pinealocytes increased; decreasing in both hypothalamic nuclei. After pinealectomy the hypothalamic nuclei showed an increase of type I neurones, but a decrease of type II nerve cells. Pinealectomy followed by substitution using pineal extracts restored the number of type I and type II neurones to that normally found in the arcuate and ventromedial hypothalamic nuclei of control animals. The present investigation brings histological evidence of an influence exerted by the rat pineal gland on nuclei forming part of the hypothalamic hypophyseotropic area. The data obtained and some of the literature strongly suggest that the type II neurones, which probably contain serotonin, inhibit, in the same hypothalamic nuclei, the production of luteinizing hormone-releasing factor (LH-RF). As yet, the function of the autofluorescent compound present in the type I neurones is not known.