A working hypothesis for the regulation of LH secretion in the mare is postulated which involves the following two components: (1) a primary central nervous system (CNS)-pituitary component which is responsible for a basal circannual LH rhythm, entrained to an environmental 'Zeitgeber' (most probably photoperiod) and independent of ovarian influences, and (2) a secondary ovarian (steroidal) component which modifies the primary rhythm during the ovulatory season. This hypothesis does not seem to apply in its entirety to FSH secretion; the CNS-pituitary component is demonstrable within the first year after ovariectomy in mares and then seems to disappear, and the ovarian component probably involves factors other than steroids on the basis of present evidence.