To determine the significance of the accessory reproductive fluid in the cock, semen collected from the ducts deferens was diluted with blood serum, seminal plasma or phosphate buffer and deposited in different regions of the hen's vagina. When semen was deposited in the anterior part of the vagina, both the ductal semen diluted with the biological fluids or phosphate buffer and the undiluted semen gave excellent fertility during the first week after insemination. When semen was deposited in

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... mid part of the vagina, the fertilizing ability of the ductal spermatozoa diluted with the biological fluids was greater than that of the undiluted semen. This was determined from the fertilization rate of eggs during the first and second week after insemination and from the duration of fertility. When semen was deposited in the posterior part of the vagina, differences in fertilization rate and in duration of fertility between the ductal semen diluted with the biological fluids and the undiluted semen were statistically significant. The more posterior was site of semen deposition, the more critical was the effect of the biological fluids on fertility. From these results, it is concluded that cock accessory reproductive fluid plays an important role in fertilization in chickens.