The objective of this experiment was to assess the relationship between electrical resistance of the vaginal mucosa and plasma progesterone for optimal mating time in the bitch. Eight mature beagle bitches were examined, and we observed eight times of estrus. Vaginal electric resistance was recorded weekly using a Draminski ovulation detector in anestrus, and daily in estrus. Plasma progesterone concentration was estimated by radioimmunoassay. In the bitch, incline in vaginal electric resistance (376.20 ± 105.63 units) showed a closely association with the onset of proestrus. Ovulation day was determined as the first day when plasma progesterone concentration increased above 5.0 ng/ml (Day 0). On Day 0, vaginal mucous electric resistance was 438 ± 132 units. Vaginal mucous electric resistance showed a slight decrease or was maintained until Day 0. However, it showed an explosive increase, and peaked on Day 1~3, which was above 600 units. Two of eight cases peaked on Day 1, three of eight cases were revealed on Day 2, and others were revealed on Day 3. After Day 4, resistance showed a rapid drop to below 600 units and reached 200 units on Day 8. The optimal mating time was determined when vaginal mucous electric resistance was above 600 units.