Ultrasonography of rabbit kidney was erformed after induction of simple and infected hydronephrosis toevaluate the sequential sonographic changes in 27 rabbits. Simple hydronephrosis was induced by ligation of thedistal ureter and infected hydronephrosis by ligation of the distal ureter and ureteral inoculation of Escherichiacoli. Ultrasonography was performed daily during the first week and weekly during the following 5 weeks afterinductin of simple and infected hydronephrosis. The results are as follows; 1. In simple hydronephrosis, theearliest abnormal ultrasonographic finding was splitting of central renal echo complex, which appeared within 1day after ureteral ligation in all cases. 2. In simple hydronephrosis, complete loss of central renal echo complexand cystic dilatation of pelvis were seen with in 5 days after ureteral ligation in all cases. 3. In infectedhydronephrosis, the earliest abnormal ultrasonographic finding was appearance of internal lapse of time and theentire pelvis was filled with internal echoes within 2 weeks after inoculation in all cases. 5. In infectedhydronephrosis, echogenecity of internal echoes within the pelvis was similar to that of renal parenchma in thefirst week after inoculation, however was weaker than that of renal parenchyma 2 weeks after inoculation in allcases.