Spontaneous right hydronephrosis occurs in approximately half the animals in an inbred strain of Slonaker-Addis albino rats. The lesion disappears in female rats as they exceed 200 g weight. The abnormality is produced by partial constriction of the right ureter by the ovarian or spermatic artery. Hydronephrosis does not appear to predispose the animals to spontaneous infection in the abnormal kidney.