A method is described for producing unisexual infections of N. brasiliensis in rats; this method was used primarily to obtain populations of virgin adult females. Experiments were carried out in which male worms were introduced to virgin females in the rat small intestine. The worms were recovered after periods varying from 30 min to 22 h and the females were examined for fertile egg production or other evidence of insemination.

In some experiments a few females were inseminated within 30 min of being mixed with males, but for reliable results a period of over 1 h was necessary. Sperm were first deposited along the ventral wall of the vagina and there was evidence that a single insemination could fill the vagina. After 4·5 h the majority of females had been inseminated and this period was long enough for sperm to reach the seminal receptacle in one female. A minority of females had a few fertile eggs in the uterus by 5·5 h, and fertile eggs were being passed by 6·0 h after the introduction of males.

Small numbers of males were introduced to an excess of virgin females in order to obtain information on the sexual capacity of males. These experiments indicated that males could, on average, fertilize one female every 2 h during the first 15–20 h. This potential was shown to be greatly in excess of the minimum requirements for maintaining the maximum fertile egg output of females in normal infections.