The research plan is designed to determine the oral bioavailability of furosemide (Lasix (R)) in human premature and term neonates as well as in infants. Using a technique of high pressure liquid chromatography, 3 microliter samples of plasma or urine can be quantitatively analyzed for furosemide. Enzymatic digests of urine are expected to yield information on glucuronide and sulfate metabolites. Using area under curve comparisons for intravenous and oral furosemide, systemic bioavailability will be estimated. Determination of protein binding and its effect upon distribution space as a variable in neonates along with quantitative 24-hour urine collections will aid in estimation of factors such as first pass hepatic metabolism which may affect the evaluation of oral absorption. Longitudinal studies of neonates into infancy with analysis of such environmental factors as diet and physiological factors such as gestational age, postnatal age, and body weight may yield information of a developmental nature as to polar drug absorption. Infants to be studied will be receiving furosemide for a medical indicator. These indications will include: bronchopulmonary dysplasia, congenital heart disease, renal failure, and hypertension. Infants to be studied will be admitted to the neonatal clinical research center at this hospital.