The main changes taking place in the female genitalia of the British Greater and Lesser Horse-shoe Bats (Rhinolophus ferrum-equinum insulanus and R. hippo-sideros minutus) during the sexual cycle are described from a rather small series of material collected throughout the year in the Bristol district.

A brief description of the structure of the genitalia is given.

The single cestrous cycle in the female takes a complete year for its accomplishment, owing to its prolongation during hibernation. There is no ancestrus apart from the lactation ancestrus.

Spermatozoa received in the autumn are stored in the fallopian tubes, uterus, and vagina of the female until spring.

During hibernation the enlarged vagina of the female is filled by a hyaline plug, the centre of which contains a core consisting of a mass of spermatozoa.

The ovum is fertilised by one of the spermatozoa stored in the higher parts of the genital tract. The corpus luteum and blastocyst are developed before the plug disappears.

The presence of the plug, which precludes the possibility of copulation, until after the formation of the corpus luteum, shows that the ovum is fertilized by a spermatozoon stored in the upper genital tract for at least five months.

The right ovary alone is functional, the left one never ovulating and rarely maturing follicles. The degeneration of the left ovary is carried further in R. Mpposideros minutus than in R. ferrum-equinum insulanus.

One follicle only is matured and ruptured each year, in the right ovary. The corpus luteum is very large and, in R. Mpposideros minutus at least, appears to be always external to the ovary and attached to it by a peduncle. It is suggested that the pedunculated corpus luteum is the result of an outfolding of the follicular wall after rupture.

The changes taking place in the fallopian tubes, uterus, and vagina are described.

In the mature animal the vaginal epithelium is always strongly cornified except during the later stages of pregnancy and lactation. The cornified epithelium adheres to the vaginal plug, which is derived from the urethral gland of the male.