The functional significance of citrate in the mammary secretion of six sows was investigated during the second stage of lactogenesis (lactogenesis II) and established lactation. The changes in the concentrations of progesterone and lactose in the maternal blood, and lactose, Na and K in the mammary secretion, suggested that lactogenesis II began during the final day of pregnancy, The concentration of citrate in the mammary secretion of the sows during lactogenesis II was high and varied from 5.4 (SEM 0.5) mm at day 0.5 post partum to 6.8 (SEM 0.4) mm at day 1.5 post partum. There was a decline of approximately 30% in the concentration of citrate in the milk of sows during the first week of lactation. These findings suggest that, in contrast to all other species studied previously, milk citrate is not a harbinger of lactogenesis II in the sow. However, the changes in the concentration of citrate in the mammary secretion of sows may reflect changes in the rate of de novo synthesis of fatty acids that take place in the mammary glands of sows during lactogenesis II and established lactation.