Yearly in the United States over 500,000 newborns are delivered prematurely. This population is at high risk of catastrophic bowel disease known as necrotizing enterocolitis. Infants with necrotizing enterocolitis are at high risk of death, and survivors are at increased risk of mental retardation. Metronidazole is an antibiotic that is often administered to infants with suspected or confirmed necrotizing enterocolitis. Unfortunately, the appropriate dose of metronidazole in premature infants has not been established and it is likely to be different from older children and adults.

The investigators will investigate the appropriate metronidazole dose in very premature infants by: 1) determining how premature infants eliminate metronidazole from the body and 2) determining the safest and most effective dose of metronidazole in premature infants.

The investigators hypothesis are: 1) The rate of removal of metronidazole will increase with infant maturity and 2) an appropriate metronidazole dosing regimen will result in necessary drug levels to treat bacteria involved in necrotizing enterocolitis.

Inclusion Criteria:

• Gestational age <32 weeks at the time of enrollment.
• Postnatal age <91 days at the time of enrollment.
• Sufficient venous access to permit administration of study medication.
• Infant suspected to have a serious infection and from whom a blood culture has been obtained within 96 hours of study entry.

Exclusion Criteria:

• History of anaphylaxis to metronidazole or other nitroimidazole derivatives (e.g., tinidazole).
• Previous exposure to metronidazole in the week prior to study.
• Previous participation in the study.

From first dose through 10 days post dose.