On the basis of a previous study showing the effectiveness of an antiprogestin treatment on the involution of the pyometric uterus and general health of bitches, in the present open clinical study seven bitches with pyometra and progesterone concentrations > or = 2 ng ml-1 were treated with the antiprogestin RU 46534. The dose selected was 5 or 6 mg kg-1 body mass s.c., on the first day of treatment and 3 mg kg-1 body mass on days 2, 3, 4, 8, 12 and 16. Antibiotics were administered until day 16. A vulval discharge was observed within 12-24 h. In one bitch the dose of the antiprogestin had to be increased after day 4. In six bitches the uterine lumen became ultrasonographically undetectable between days 8 to 12; in one bitch some luminal material could still be detected on day 28. The number of blood leucocytes tended to increase after the onset of treatment but had returned to the upper normal range by day 16. In all dogs general condition and feed consumption improved rapidly and were normal within 8 days. No side effects were noted. Two of the dogs were mated subsequently and produced two healthy litters. These observations confirm those of a preceding study and show that the treatment of pyometra with an antiprogestin may lead to a clinical recovery. These results justify further studies to enable the optimum treatment regimen to be established.