In order to investigate the severity and incidence of hyperammonemia in preterm infants receiving total parenteral nutrition (TPN) with crystalline L-amino acids having high arginine content (Travasol), we determined the plasma ammonia (PA) levels in a group of 29 preterm infants on TPN, weekly and 1 wk posttherapy. Their mean gestational age was 29.9 +/- 2.6 wk and mean birth weight 1208 +/- 262 g. Thirty five blood samples obtained from 15 preterm infants not on TPN with mean gestational age 32.2 +/- 1.9 wk and a birth weight of 1495 +/- 161 g served as a control. In the parenteral nutrition group the mean PA level (140 +/- 58 micrograms/100 ml) was significantly higher (p less than 0.001) than that in the same group one week post TPN (97 +/- 34 micrograms/100 ml) and in the control group (86 +/- 35 micrograms/100 ml). The incidence of hyperammonemia (greater than 160 micrograms/100 ml) was 30% in the TPN group versus 3% in the controls (p less than 0.01). Maximal PA level during that treatment was 405 versus 216 micrograms/100 ml 1 wk post-TPN versus 163 micrograms/100 ml in the controls. The data show a significant increase in PA levels in preterm infants receiving TPN with Travasol, possibly because of its high glycine content.