Complete obstruction of one ureter was created in 3-week-old weanling rats. The endothelin concentration in the renal tissue was measured by radioimmunoassay. In the obstructed kidney, a substantial increase was observed, after 1 week +90%, after 2 weeks +55%, and after 4 weeks + 145% compared to the control rat kidneys. The endothelin-l/endothelin-3 ratio was found to be considerably raised, indicating a predominance of the vasoconstrictor effects of the – with reference to vasoactivity – bi-potential endothelin. Its contribution to vasoconstriction and to glomerular destruction in obstructive nephropathy is discussed.