1. Reproduction in the water-vole, Arvicola amphibius, is described from field material comprising 128 males and 95 females.

2. The reproductive organs resemble those of the bank-vole.

3. The breeding season extends from the end of March to the latter half of September, but fecund males were caught in February.

4. Animals born early in the year breed in the same season. The average number of corpora lutea of pregnancy was 6-4 and of embryos in utero 5–7.

5. Males are non-fecund in winter, when the testes are very small; the testes of non-fecund adults can be distinguished from those of immature animals by the condition of the tunica albuginea.

6. The female has a poly-cestrous cycle, and both non-parous animals and parous animals at the beginning of their second year may undergo infertile cycles. (Estrus is accompanied by distension of the uterus with fluid as in the rat and mouse.

7. There is a post-partum oestrus, and several animals were found to be pregnant and lactating. Animals which fail to become pregnant at the postpartum oestrus enter upon a lactation anoestrus. This may be followed by another oestrus, but late in the season the animal passes from lactation anoestrus to winter anoestrus without ovulating again.

8. The ovarian follicles reach about 700/J. in diameter at oestrus, but large follicles are also found at all stages of pregnancy. The corpora lutea reach their maximum size, about 1–7 mm. in diameter, fairly late in pregnancy.

9. Cyclical changes in the Fallopian tubes and uterus are described.

10. The mammary glands afford good evidence of the condition of the animal and make it possible to distinguish between the first pregnancy of the season and a subsequent pregnancy.

The material described above was obtained in 1932-35 as part of a general programme of work on the reproductive processes in British mammals, and was made available to me by Dr. A. S. Parkes, who suggested this work.