AbstractChlorcyclizine was given to pregnant rats in daily doses of 60 or 80 mg/kg on days 12‐16 of gestation. The lower dose induced hydronephrosis and hydroureter in 37% of fetuses and undescended testes in 55% of the males whereas at the higher dose only 4% were hydronephrotic although 100% of the males were cryptorchid. Histological examination revealed the hydronephrosis and hydroureter to be secondary to either ureteral aplasia or persistence of the ureteropelvic membrane. The same membrane was observed in fetuses exposed to the higher dose although hydronephrosis was infrequent in these animals. Sections revealed dysplasia, tubular disorganization, and increased connective tissue in most kidneys from fetuses treated with the higher dose. A hypothesis to explain these findings is that the ureteropelvic membrane or atresia is present at both doses but urine production is reduced at the higher dose and thus no hydronephrosis results.