During pregnancy a higher amino acid requirement may be expected, but the increase in food intake does not match the increased growth rate during pregnancy. It is hypothesized that amino acid utilization can be increased during both fasting and feeding in order to account for the increased requirement. Therefore mature female rats (20 weeks old) were investigated before and at day 18 of pregnancy. Rats were fed on a high-protein (HP) diet (210 g casein/kg diet) for 3 weeks and fasted overnight. Rats were then subjected to an 8 h constant infusion of L-[1-14C]leucine with continuous measurement of expired 14CO2 (as a percentage of the infused dose). After 3 h infusion a 5 g HP or low-protein (LP; 75 g casein/kg diet) meal was offered for 30 min. Pregnant rats had a significantly lower percentage leucine oxidation in the fasted state (12.5 (SE 0.7) v. 15.9 (SE 1.1)%; P < 0.05), which suggests improved reutilization of leucine. Meal ingestion resulted in a fast increase in 14CO2 expiration. After the LP meal the level of 14CO2 expiration decreased again after the acute response (0-1.5 h), but this was not the case after the HP meal. After the HP meal (average 1.5-5 h), no difference was observed between pregnant and non-pregnant status (36.8 (SE 1.6) v. 35.0 (SE 2.5)%). After the LP meal (average 1.5-5 h), however, the percentage leucine oxidation tended to be lower in pregnant rats but this difference did not reach statistical significance (19.7 (SE 1.1) v. 25.8 (SE 2.8)%).(ABSTRACT TRUNCATED AT 250 WORDS)