Male and female mice were anesthetized by intraperitoneal injection with a mixture delivering 0.5 mg/kg medetomidine and 50 mg/kg ketamine to achieve immobilization for whole-body radiographs and bone densitometry, as part of a phenotypic screen for bone and mineral disorders in mice carrying genetic modifications induced through mutagenesis with N'-ethyl-N'-nitrosourea. Morbidity and mortality occurred in 19 of 628 (3%) of male mice 24 to 72 h after a seemingly uneventful recovery from anesthesia. No morbidity or mortality occurred in 1564 female mice that were similar in age to the affected male mice and that underwent the same procedure. Of the 7 male mice that underwent postmortem examinations, 5 had urinary bladders grossly distended with urine and 1 had ascites. In addition, the pelvic or penile urethra in 5 of the examined male mice was obstructed with seminal coagulum associated with varying degrees of erosion of the urothelial lining and inflammation of the urethra. In 2 of these animals, from which plasma samples were recovered, azotemia, hyperphosphatemia, and hyperkalemia were present. The predilection for delayed morbidity and mortality in males after anesthesia suggests that anesthesia with 0.5 mg/kg medetomidine and 50 mg/kg ketamine is a potential risk factor for obstructive uropathy due to release of seminal coagulum. This adverse effect did not recur when we altered our anesthesia protocol to 10 mg/kg xylazine and 100 mg/kg ketamine.