The present study was undertaken to define the possible role of EOLS in controlling renal functions. in mediating the renal responses to furosemide(F) and in inducing endocrine reactions in F-treated neonates. Ten newborn infants with mean birthweight of 2752 g and mean gestational age of 37.1 weeks were given F in a dose of 1 mg/kg. Prior to and following F therapy urine was collected for a period of 12 hours and analyzed for creatinine, osmolality, Na and K, as well as for EOLS, AVP, aldosterone and ET-1. In response to F urine flow rate and urinary osmolar, sodium and potassium excretion increased significantly, whereas creatinine excretion remained unchanged. Furthermore, following F therapy there was an increase in EOLS (148.7±70.8 vs 200.1±98.1 pg/kg/h, p<0.01), ET-1 (36.0±5.6 vs 61.4±8.7 fmol/kg/h. p<0.001), AVP (19.5±5.4 vs 27.8±7.8 pg/kg/h, p<0.05) and aldosterone (507±120 vs 751±203 ng/kg/h, p<0.05) excretion, respectively. Prior to F urinary EOLS excretion was found to correlate positively with diuresis, Na, creatinine, osmolar, ET-1 and AVP excretion. After F urinary EOLS excretion significantly correlated only with diuresis ET-1 and AVP. Urinary EOLS proved to be independent of aldosterone excretion irrespective of F administration. It is concluded that urinary EOLS excretion is closely related to neonatal renal functions and it may have a role in controlling diuresis and natriuresis.