Five multiparous and four nulliparous bitches had the electrical resistance of vaginal secretions measured by an ohmmeter. The bitches were examined daily, starting on the first day of vulval bleeding and continuing until the first day when exfoliated vaginal epithelial cells became predominantly noncornified. In contrast to cattle, sheep, and swine, the electrical resistance of the vaginal secretions in the bitch increases during pro-oestrus and continues to be high while the bitch is showing oestrous behaviour and a high percentage of eosinophilic superficial cells in the vaginal smear. Ovulation, as determined by a rise of serum progesterone above 5 ng/ml, occurs within the period of elevated resistance. Optimal conditions for conceptions coincide with the last 3 d of elevated resistance.
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