The protein binding of furosemide was studied in the serum from 8 umbilical cords, in 51 children (aged between 2 weeks and 13.5 years) and in the plasma of 10 volunteers (aged between 28 and 42 years). The drug was added to the buffer to give a final concentration of 2 micrograms/ml. The unbound fraction of furosemide was 2.5 +/- 0.1% (cord serum) and 1.7 +/- 0.7% (adult plasma). These figures are different at a level of 0.001. The unbound fraction of furosemide reached the adult values during the 1st year of life. A correlation (level of significance greater than 0.01) was found between the unbound fraction and the age during the first 6 months of life. The furosemide binding kinetics were studied in 3 cord serum and in 3 adult plasma samples. The concentration of the drug in the buffer ranged between 1 and 16 micrograms/ml. The kinetic constants (mean +/- SEM) were: association constant (K = 10(5) M-1) 2.4 +/- 0.3 (cord serum), 2.0 +/- 0.2 (adult plasma); the number of binding sites per gram protein (n = 10(-6] was 3.2 +/- 0.5 (cord serum) and 3.9 +/- 0.7 (adult plasma). When the concentration of furosemide was increased up to 200 micrograms/ml buffer, the free fraction of the drug increased up to 4.8 +/- 0.2% (cord serum) and 2.9 +/- 0.4% (adult plasma).