Resting pituitary melanocyte-stimulating hormone (MSH) content was studied in adult rats kept under a controlled 14 h light-10 h dark regimen. A diurnal rhythm was found in pituitary MSH levels, showing peak values at 4.00–6.00 p.m., and low values during darkness and the beginning of the light period. The normally occurring fall in pituitary MSH content between 6.00 p.m. and midnight was also found in rats exposed to a prolonged photoperiod. Sympathetic denervation of the pineal gland by superior cervical ganglionectomy, as well as pinealectomy, did not block the rhythm in pituitary MSH, and had no effect on pituitary MSH content. In addition, melatonin administered i.v. did not affect pituitary MSH levels, either when MSH activity was tested on the Anolis skin in vitro, or when the in vivo assay with hypophysectomized frogs (Rana pipiens) was used. It was concluded that the diurnal rhythm in pituitary MSH content is not directly related to the light-dark cycle and is not controlled by pineal melatonin or any other pineal facto