The postnatal development of the urea-synthesizing capacity was studied in 21 preterm infants with intrauterine growth retardation (IUGR) and compared with results found in 12 infants without IUGR as controls. The urea-synthesizing capacity was estimated by the ratio Q of 15N abundance of ammonia and urea in 6-hour urine samples collected after enteral administration of 3 mg [15N]H4Cl/kg body weight. The measurements were performed on the first day when a protein intake of 3.0-3.5 g/kg/day and an energy intake of 120 kcal/kg/day were tolerated (study day 1: postnatal 14-21 days) and on the day of discharge from the hospital (study day 2: postnatal age 39-56 days). The group of infants with IUGR was subdivided in one group of infants who developed catch-up growth (n = 12) and one group who did not demonstrate catch-up growth (n = 9). On study day 1, the Q values of the IUGR infants without catch-up growth were significantly higher than those of the IUGR infants with catch-up growth (13.4 +/- 2.3 vs. 9.2 +/- 2.2) or of the control infants without IUGR. During the time period from study day 1 to study day 2 the Q values of the IUGR infants with catch-up growth decreased significantly (9.2 +/- 2.2 vs. 4.8 +/- 2.0; p < 0.001) and were in the range of the control infants without IUGR. In contrast, the Q values of the IUGR infants without catch-up growth did not significantly change during the study period (13.4 +/- 2.3 vs. 11.3 +/- 2.8; p = 0.097). On both study days there was a significant correlation between the Q values and the degree of IUGR (study day 1: r = 0.652, p < 0.01; study day 2: r = 0.842, p < 0.001). The data indicate that the urea-synthesizing capacity of preterm infants increases during early postnatal life and that severe IUGR may impair this development. Metabolic investigations using urea as marker for evaluation of optimal quantity or quality of dietary proteins should carefully be interpreted when infants with severe IUGR are studied.