The objective of this study was to evaluate the clinical efficacy of three different doses of aglepristone administered in early diestrus for prevention of pregnancy in mismated dogs. A total of 48 mismated, copulation confirmed bitches were used as material. Bitches were randomly allocated into one of four groups and treatment groups received 5 mg/kg (group I, n=12), 7.5 mg/kg (group II, n=12) or 10 mg/kg (group III, n=12) of aglepristone by two injections, 24 h apart within 10 days of diestrus. Twelve bitches (group IV) served as controls. Pregnancy was assessed by ultrasonography on day 30 of diestrus. Blood samples were collected for determination of plasma progesterone concentrations by radioimmunoassay on days 0, 3 and 7 of first aglepristone administration and on the day of ultrasonographic examination. None of aglepristone treated bitches except for one bitch in group I became pregnant. Pregnancy rate in the control group was 83.3%. There was no significant difference between mean plasma progesterone concentrations in study groups on day 0 and 3 (P>0.05). On day 30 of diestrus, plasma concentrations of progesterone were significantly lower in aglepristone treated bitches (group I: 92.60±5.74 nmol/L; group II: 85.66±3.69 nmol/L; group III: 78.87±4.86 nmol/L) compared to controls (110.62±6.24 nmol/L, P<0.01). Aglepristone administration resulted in shortening of interestrus interval in all groups (group I: 15.33±5.29 days; group II: 12.78±4.94 days and group III: 14.45±4.29 days, P>0.05). Twenty aglepristone treated bitches were bred in the subsequent estrus and overall pregnancy rate obtained was 90%. In conclusion, aglepristone at lower-than-standard doses administered in early diestrus effectively prevents pregnancy in mismated bitches and potentially offers a cost-effective treatment option.