Abstract Several of the factors governing transcapillary fluid balance were studied in anesthetized rats from the age of 1 to 60 days. Serum albumin and total protein concentrations rose from 1.7 and 2.4 g/100 ml at birth to 4.1 and 6.2 g/100 ml in adult rats, while colloid osmotic pressure rose from 5.3 to about 20 mm Hg. Interstitial fluid collected from subcutis by the wick technique showed protein concentrations of approximately 60% of serum values in all age groups, and its colloid osmotic pressure rose from about 3 to 10 mm Hg during maturation. Arterial pressure rose from about 50 mm Hg in newborn rats to 120 mm Hg in adult animals. Iliac venous pressure was only 0.5–1 mm Hg in 10-day-old rats compared to 3 mm Hg in adult animals. Interstitial fluid pressures of 0 to −1 mm Hg were obtained in all age groups with the “wick-in-needle” technique. The data suggest an average capillary pressure of less than 5 mm Hg in newborn animals and a pre- to postcapillary resistance ratio similar to that of adult animals. The safety factors against edema formation seem to be small in immature rats.