The effects of 24 h of fasting and refeeding on the release of oxytocin, prolactin, insulin and LH in three lactating sows were investigated. The sows were starved, but supplied with water ad libitum, from 09.00 h on day 27 of lactation until 15.00 h on day 28 of lactation, when they were refed. Blood samples were collected continuously, using an automatic collection system, at a rate of 1 ml min-1 from 09.00 to 21.00 h on day 28 (P1 = 6 h period after the 24 h fast, P2 = 6 h period after refeeding). For both P1 and P2 the mean number of nursings was 7.0 ± 1.0. Plasma insulin and glucose decreased to very low levels during fasting and increased (P < 0.001) after refeeding (insulin, 2.5 ± 0.7 vs. 28.9 ± 0.7 mU 1-1; glucose, 2.6 ± 0.3 vs. 6.4 ± 0.3 mmol 1-1). Following fasting, levels of prolactin were low (2.8 ± 0.1 μg 1-1), and sucking did not induce significant release of prolactin. However, prolactin increased rapidly after refeeding (5.4 ± 0.1 μg 1-1, P < 0.001). Neither the 24 h fast nor refeeding had a marked effect on basal levels of oxytocin, the percentage of sucklings with an oxytocin peak or the size of oxytocin peak. LH release (average and basal levels and number of pulses/6 h) during fasting was similar to that measured after refeeding. Plasma CCK increased significantly after feeding. The results indicate that the release of prolactin is also regulated by feed intake. Consequently the 24 h fast in lactating sows may alter milk production via an inhibition of prolactin release. Neither a stimulatory effect of insulin nor an inhibitory effect of prolactin on LH release was observed.