Hypothalamic regulation PRL secretion was investigated in 13 7-month-old Yorkshire gilts by comparing the effects of hypophysial stalk transection and sham operation. Ovariectomized gilts were fitted with an indwelling cannula in the anterior vena cava to determine sequential serum profiles of PRL secretion before, during, and 190 h after cranial surgery. A nylon disc was inserted between severed ends of the hypophysial stalk to prevent vascular and tissue regeneration; sham operations included all surgical procedures with exception of stalk transection and insertion of the disc. During a preoperative period of 120 min, PRL concentrations in peripheral serum remained consistently low [2.3 +/- 0.3 ng/ml (mean +/- SEM)] in all gilts. During 105 min of anesthesia (induced by thiamylal sodium and maintained by halothane and oxygen), PRL increased (8.2 +/- 1.9 ng/ml) in all gilts. Peak PRL values averaged 10.7 ng/ml at hypophysial stalk transection or sham operation and then declined steadily in both groups during the last 105 min of surgery. PRL remained elevated (P less than 0.002) in hypophysial stalk-transected gilts compared with sham-operated controls throughout a postoperative period from 6-190 h (3.7 +/- 0.4 vs. 1.9 +/- 0.2 ng/ml). These results indicate that basal secretion of PRL in the pig is inhibited by the hypothalamus.