Little is known about the response of the bitch's reproductive tract to mating or of the role of male accessory gland secretions in the female. In this clinical study, the component stimuli causing mating-induced uterine contractions were investigated in 64 bitches. Basal uterine contractions were present during oestrus and a significant increase in the frequency of contractions was observed during natural mating. Neither teasing with a male nor stimulation of the vagina or cervix by vaginal or transcervical insemination (TCI) caused an increase in the frequency of uterine contractions. Increased contractions were however present after both vaginal and transcervical insemination when the vestibule was distended, and dorsal wall of the vaginal was manually stimulated. Interestingly, this increase in uterine contractions was partially ameliorated when prostatic fluid was used as a flushing component following transcervical insemination. Two further studies performed with 72 bitches of which 18 were each inseminated transcervically with fresh or frozen semen flushed into the uterus with either saline or prostatic fluid demonstrated that prostatic fluid significantly increased the pregnancy rate and litter size of both groups. There are important mechanisms regulating the transport and elimination of sperm from the bitch's reproductive tract. Whilst physical aspects of coitus are undoubtedly involved in initiating uterine contractions, prostatic fluid appears to have an important role in modulating uterine contractions and fertility.