On differences between the preovulatory luteinizing hormone surges of 4- and 5-day cyclic rats.

Preovulatory LH surges were studied in rats with ovarian cycles of 4 days (4d-rats) and 5 days (5d-rats). In 5d-rats the maximal peak height was about twice that observed in 4d-rats, whereas in 4d-rats peaks occurred about 1-5 h later than in 5d-rats and were much less consistently timed. From experiments in which LH releasing hormone (LH-RH) was infused into pentobarbitone-blocked pro-oestrous rats, it was concluded that (a) differences between the two types of preovulatory LH surges originate within the central nervous system, and (b) pro-oestrous LH-RH secretion may not be restricted to the period in which increased LH levels in blood were found, but may exceed this period for a considerable time.