The reproduction of the Field Vole, Microtus agrestis hirtus (Bellamy), was investigated. The sex-ratio was found to be 50.45±2.26 males per cent. The breeding season extends from the middle of March to September or even October, but males are fecund a month earlier in spring. Animals born early in the year breed during the same season, when only a few weeks old. Animals born late in the season do not attain puberty until the following year. The mean number of ova ovulated at oestrus is 5.5; the mean number of embryos in utero is 5.0. The female has a polycestrous cycle. Young females frequently experience one or more diœstrous cycles after puberty before becoming pregnant. A post partum oestrus follows parturition; many animals then becoming pregnant although lactating. Animals which do not become pregnant at the post partum oestrus enter a period of ancestrus while lactating, which may be followed by the recurrence of oestrus.

We wish to express our thanks to Mr. D. H. S. Davis for providing us with the 22 females of M. agrestis neglectus (Thompson) and to all those who assisted in the collection of the material. Our thanks are due to the Rt. Hon. Lord Penrhyn for permission to trap on his estates. The expenses of this research were defrayed in part by a grant from the Government Grant Committee of the Royal Society to one of us (F, W. R, B.), which we gratefully acknowledge.