We measured plasma endothelin-1-like immunoreactivity (ET-1-LI) levels in the peripheral vein of 70 healthy children (37 males and 33 females) aged 5 days to 15 years to establish the normal range in children, and compared them with those in 11 normal adults (age, mean +/- SD; 29.5 +/- 5.7 years, 6 males and 5 females). Plasma ET-1-LI levels (pg/ml, mean +/- SD) in children of each age group and adults were as follows: children younger than 1 month = 23.4 +/- 3.9 (n = 10), 1-3 months = 25.0 +/- 6.1 (n = 8), 3-6 months = 17.0 +/- 1.4 (n = 8), 6 months to 1 year = 14.0 +/- 1.7 (n = 13), 1-5 years = 13.8 +/- 2.6 (n = 11), 5-10 years = 13.6 +/- 1.6 (n = 10), 10-15 years = 12.9 +/- 2.2 (n = 10), and adults = 13.2 +/- 2.4 (n = 11). There was no significant difference in plasma ET-1-LI levels between males and females in each age group. Plasma ET-1-LI levels in children younger than 3 months were significantly (p less than 0.01) higher than those in older children or adults. After 3 months of age, plasma ET-1-LI levels were nearly constant at all ages. This is the first report to establish the normal range of plasma ET-1-LI levels in children.