The effect of adrenocorticotropic hormone (ACTH) administration on progesterone and cortisol concentrations was determined in lactating primiparous sows. Physiological saline and 50 IU ACTH were administered on days 20 and 21 of lactation at 0900 hours via an indwelling jugular catheter. Blood samples for hormone analysis were collected via indwelling jugular catheters every 15 min (0800 to 1200 hours) and every 60 min (1300 to 1500 hours). Saline administration had no effect on progesterone nor cortisol concentrations in the lactating sows. Progesterone and cortisol concentrations increased (p < 0.001) within 15 min after ACTH administration. Progesterone and cortisol concentrations peaked (p < 0.01) within 45 min and had returned to pretreatment values within 120 min after ACTH treatment.