The purpose of this study was to determine whether intravaginal prostaglandin F2α(PGF2α) would be effective for the treatment of metritis or pyometra in the bitch. Seventeen bitches with metritis or pyometra were treated with PGF2α. Prostaglandin F2α(150 (g/kg body weight) was administered once or twice daily by infusing 0.3 ml per 10 kg body wt into the vaginal lumen. Bitches were also treated with amoxicillin (15 mg/kg body wt/48 h) and/or gentamicin (4 mg/kg body wt/day) administered as intramuscular (i.m.) injections. Fifteen bitches were treated successfully with intravaginally administered PGF2αfor 3 to 12 days and with intramuscularly administered antibiotics for 4 to 12 days. Success of treatment was judged by cessation of vaginal discharge, the absence of fluid in the uterus as determined by ultrasonography, and the overall health status of the animal. As two bitches with pyometra showed clinical deterioration in spite of medical treatment, ovariohysterectomy was performed after the first and the second treatment, respectively. No side effects (salivation, vomiting, diarrhoea, hyperpnoea, ataxia, urination, anxiety, pupillary dilatation followed by contraction) were observed after PGF2αtreatment. The disease did not recur during the subsequent oestrous cycles within 12 months after the initial treatment. The results demonstrate that intravaginal administration of PGF2αwas effective in 13 dogs (86.6%) with metritis or pyometra, and caused no side effects. Although the study was based on a relatively small number of cases, it is concluded that prostaglandin F2αcan be a useful means of treating bitches with metritis or pyometra. However, in severe cases of pyometra ovariohysterectomy is needed.