Arteriovenous difference studies across the lactating rabbit mammary gland for glucose, acetate, triacylglycerol and non-esterified fatty acids during initiated involution are reported. 2. A significant reduction in substrate utilisation is paralleled by a decrease in the activities of fatty acid synthetase, acetyl CoA synthetase, citrate synthase and glutamate dehydrogenase in biopsy samples taken from the gland. 3. Results from the analysis of lipid fractions within the gland during this period are discussed in relation to lipid resorption.