The corpus luteum (CL) is a transient endocrine gland that develops from a graafian follicle after ovulation and is required to support pregnancy in mammals. LH is essential for maintenance of progesterone production by luteal cells of many species. The concept that LH is the main luteotro-pic hormone in cattle was proposed over three decades ago. In cattle, there is a greater frequency of episodic LH pulses during the follicular phase (~7 pulses per 12 h) as compared with the midluteal phase (~3 pulses per 12 h) of the estrous cycle. In cattle, association between LH pulses and secretion of progesterone indicates that during the midluteal phase of the estrous cycle, LH pulses are followed by a release of progesterone; however, pulses of LH and progesterone secretion are not associated during the early luteal phase.
Dependence of luteal function on LH varies during different stages of the estrous cycle and among species. Abolishment of LH pulses by treatment of ewes with an LHRH antagonist on Day 6 of the estrous cycle (Day 0 = estrus) resulted in a slight decrease in concentrations of progesterone in blood, and concentrations remained suppressed until the typical time during the estrous cycle for cessation of luteal function.