Abstract
Luteinizing hormone (LH) activity in blood was assessed by the ovarian cholesterol depletion (OCD) method and oestrogens were estimated chemically in the urine of 7 sows before and during estrus. In 3 sows, peritoneal cannulation permitted the time of ovulation to be related to LH and estrogen determinations. The findings suggest a marked elevation of blood LH activity coincident with a peak of urinary estrogen excretion, and occurring 40 to 48 hr. before the time of ovulation.