Ultrasonography of rabbit kidney was erformed after induction of simple and infected hydronephrosis to evaluate the sequential sonographic changes in 27 rabbits. Simple hydronephrosis was induced by ligation of the distal ureter and infected hydronephrosis by ligation of the distal ureter and ureteral inoculation of Escherichia coli: Ultrasonography was performed daily dudring the first week and weekly during hte follows; 1. In simple hydronephrosis, the earilest abnormal ultrasonographic finding was splitting of central renal echo complex, which appeared within 1 day after ureteral ligation in all cases. 2. in simple hydronephrosis, complete loss of central renal echo complex and cystic dilatatioin of pelvis were seen with in 5 days after ureteral ligation in all cases. 3. In infected hydronephrosis., the earliest abnormal ultransonographic finding was appearance of internal echoses in dependent portion of the pelvis, which appeared within 4 days after inoculation in all cases. 4. In infected hydronephrosis, degree of internal echoes within the pelvis increased progressively with lapse of time and the entire pelvis was filled with internal echoes 2 weeks after inoculation in all cases. 5. In infected hydronephrosis, echogenecity of internal echoes withn the pelvis was similar to that of renal parenchma in the first week after inoculation, however was weaker than that of renal parenchyma 2 weeks after inoculation in all cases.