A new sampling method for the collection of fore and hind milk from the sow has been developed which resembles normal milk removal by the piglet, yet overcomes the difficulties of collecting milk that is available for only a short time (10-20 s) at each let-down. Samples of hind milk were collected immediately after the completion of a successful sucking, while the fore milk was collected at the beginning of the next natural let-down. Modification of existing assays for fat, protein and lactose was required to provide rapid analysis of the small volumes (<0.5 ml) of milk collected using this procedure, and these methods were validated by comparison with reference methods. Total solids and the concentration of fat in whole milk, and protein and lactose in skim milk, were measured in fore and hind milk collected from 4, 20, 12 and 12 sows respectively, 6-11 d post partum. For fore milk, the results (mean+/-SD (n)) were: total solids, 199.9+/-9.9 g/l (8); fat, 96.7+/-13.9 g/l (41), protein, 47.7+/-4.5 g/l (27) and lactose, 58.3+/-4.0 g/l (27). In hind milk, there was a significant increase in the concentration of fat (15.3 g/l, P <0.001, n = 41) which was reflected by a significant increase in total solids (14.7 g/l, P <0.001, n = 8) and calculated energy (511 kJ/l, P <0.001, n = 27), but there was no significant difference in the concentration of either protein or lactose. This increase in milk fat during milk let-down is in contrast to the results of most previous studies in the sow and is discussed in terms of the possible mechanisms that may cause the concentration of fat to increase as milk is removed from the mammary gland.