Anterior pituitary and plasma prolactin in rats after 2 to 90 minutes of suckling.

Separation of litter for 6 hr resulted in plasma PRL levels of 48 +/- 5.8 and 45 +/- 5.7 ng per ml and AP PRL concentrations of 6.1 +/- 0.6 and 6.3 +/- 0.5 mug per mg AP in early (4-8 days) and late (20-22 days) lactation, respectively. After a 6-hr separation period, suckling for 2, 30, 60, and 90 min resulted in highly significant increases in the plasma PRL level in early lactaiton. Increases in plasma PRL levels were not seen in late lactation except in the 30-min suckled group. In both stages of lactation the AP PRL concentrations in the suckled groups were significantly lower than in the nonsuckled groups.