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Two experiments were conducted to monitor hormonal changes during lactation in crossbred sows (Pietrain x German Landrace). Sows were fed twice daily without weighing the remaining food. Number of piglets was not standardized. Plasma concentrations of growth hormone (GH), prolactin (PRL), insulin-like growth factor-1 (IGF-1), IGF-2, insulin (INS), triiodothyronine (T3), thyroxin (T4), free thyroxin (FT4), non esterified fatty acids (NEFA) and glucose (GLUC) were determined by RIA, EIA or enzymatically. In exp. A (n=5 sows), blood samples were taken via permanent jugular cannula in weekly 24 h windows at 20 min intervals and additionally once daily for 6 weeks during lactation and for 3 days after weaning. In exp. B (n=24 sows), blood was collected by needle puncture of the ear vein 2 and 1 week before parturition, the Ist and 3rd-4th week of lactation and 3 and 2 weeks after weaning. GH (0.8 ng/ml) and PRL (10.2 ng/ml) increased with onset of lactation (3.3 resp. 91.5 ng/ml), remained at high levels (2.5-2.8 resp. 39-41 ng/ml) during the 2nd and 3rd week, declined slowly thereafter and considerably after weaning to concentrations of 0.7 resp. 2.7 ng/ml. During lactation in 4 of 5 sows in exp. A, the typical episodic secretory pattern of GH and PRL was lost due to frequent suckling. Basal values, as known from non lactating sows, were not reached and number of pulses was elevated during lactation for both pituitary hormones. Insulin levels showed a high individual variation. Mean values did not change significantly throughout lactation (0.8-1.2 ng/ml), but tended to be lower at the end of lactation and after weaning (0.8-1.0 ng/ml). NEFA levels were highest in the 2nd week of lactation (0.9 mMol/l) and decreased steadily thereafter and after weaning (0.2-0.3 mMol/l). IGF-1 increased from 77 ng/ml 2 weeks before parturition to 144 ng/ml at parturition, remained high (467-552 ng/ml) for 4 weeks and declined to 249 ng/ml after weaning. IGF-2 increased from the 1st to the 3rd week of lactation (350 resp. 430 ng/ml), remained at that plateau and decreased after weaning (300 ng/ml). T3 (0.7-0.9 ng/ml), T4 (25-32 ng/ml) and FT4 (10-13 pg/ml) did not change. In conclusion, the secretory patterns of metabolic hormones esp. increased IGF-1 - may indicate a regulatory mechanism that can help the sow to reduce her protein mobilization during lactation.