1. Measurements were made of arterial and coccygeal concentrations of plasma constituents and of arteriovenous differences across the mammary gland in two anaesthetized lactating sows, and of coccygeal–mammary-venous differences in three conscious sows when lactating and again later when ‘dry’. 2. With the possible exception of acetate concentration, the compositions of arterial and coccygeal plasma were similar, and arteriovenous differences in the anaesthetized lactating sow corresponded closely to coccygeal–venous differences in the conscious animal. 3. In the ‘dry’ sow coccygeal–venous differences were in all instances small. 4. In the lactating sow there were large arteriovenous (or coccygeal–venous) differences (mean value as a percentage of arterial or coccygeal concentration) in glucose (31%), acetate (46%), arginine (27%), glutamate (42%), histidine (26%), isoleucine (36%), lysine (25%), leucine (39%), methionine (38%), phenylalanine (32%), proline (31%), threonine (22%), tyrosine (32%) and valine (27%), and in palmitate (19%), oleate (23%), linoleate (21%) and stearate (16%) of the plasma triglycerides. The values for the following constituents were in all instances small: β-hydroxybutyrate, acetone+acetoacetate, citrate, lactate, alanine, glycine, aspartate, palmitoleate of the plasma triglycerides, phospholipids, cholesterol, cholesteryl esters and free fatty acids. 5. Of the total recorded uptake of plasma constituents by the lactating gland, 59% was accounted for by glucose, 28% by amino acids, 11% by plasma triglycerides and 2% by acetate. The relative uptakes of glucose and amino acids were higher in the sow than values reported previously for the goat, and the relative uptakes of acetate and triglycerides much less.

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