1. Three groups of 4 sheep were penned individually and provided with diets composed of either dried grass, 80% ground maize/20% soyabean meal or 80% ground barley/20% soyabean meal. 2. The diets were fed ad libitum for 3 weeks before s.c. adipose tissue biopsy samples were taken from the rump region. 3. Although the rate of lipogenesis was significantly increased by concentrate feeding the order of utilization of the various substrates was always acetate greater than glucose greater than lactate. Throughout this work lactate was always of minor significance as a lipogenic substrate. 4. The diet-induced differences in lipogenesis were reflected in significant increases in the specific activities of the fatty acid synthetase system, glucose-6-phosphate dehydrogenase and phosphogluconate dehydrogenase in concentrate fed animals. 5. No differences were observed in in vitro lipogenesis from any of the substrates or enzyme specific activity between the 80% barley diet and the 80% maize diet. 6. These results are discussed in relation to the effect of concentrate and roughage feeding on the entry of alpha-linked glucose polymer into the small intestine of sheep.