1. Fetal lung metabolic response to maternal fasting late in gestation was investigated. 2. Maternal fasting 4 days before term was associated with low fetal plasma glucose and insulin levels but increased levels of fetal plasma glucagon, glycerol, lactate and fatty acids. 3. Fetuses from fasted mothers showed a significant decrease in body weight (30%) and lung glycogen (46%), but no change in lung protein, phospholipid or total lung DNA, suggesting that lung size is affected more than maturation. 4. Fetal lung slices incubated in vitro showed that lactate oxidation to CO2 equalled that of glucose in control fetal lungs and was unaffected by maternal fasting, while glucose oxidation was depressed (23%). 5. Maternal fasting significantly decreased in vitro incorporation of [U-14C]-glucose, [U-14C]lactate and [1-14C]palmitate into lung phospholipids. 6. Fetal lungs from fasted mothers showed increased conversion of lactate to glucose, indicating gluconeogenic potential by fetal lung. 7. These studies show that plasma lactate serves as an important energy fuel and substrate for lipid synthesis for the fetal lung, and maternal fasting markedly alters fetal lung metabolism.