(1) Gross anatomical studies have been made on the pelvic and perineal regions of some 186 ewes including representatives of the Welsh mountain, Suffolk, and Romney Marsh breeds. A few embryos and foetuses were also examined to elucidate details in the arrangement and nerve supply of certain muscles. (2) The investigations were made primarily to obtain basic information for problems of veterinary obstetrics, and the various tissues have been considered with respect to their relative importance in parturition. The findings have been discussed also in relation to the literature concerning the pelvic and perineal regions of other mammalian species. (3) The structure of the sacroiliac joint has been studied in detail, and the presence of a rudimentary joint capsule, associated with the articulating surfaces, confirmed. It has been suggested that the areas of the opposing surfaces of the sacrum and the ilium joined by fibrous tissue (known as the interosseus ligament in man) constitute a more important component of the joint than is generally appreciated. The interosseus collagenous strands of this "syndesmosal" portion of the sacroiliac joint also contain numerous elastic fibres and are interspersed with quantities of fat. These structural characteristics, considered in relation to the position of the joint in the animal's body, may indicate a shock-absorbing function for the fibrous portion of the joint. (4) The dorsal and accessory (short and long) sacroiliac ligaments, described by previous writers for ungulates, could not be found in the ewes dissected here. It is considered that these ligaments may represent thickenings of the aponeurosis of the longissimus dorsi muscle which continues on to the muscles in the sacrococcygeal region and has bony attachments similar to those described for the "short" and "long" ligaments. The attachments of the ligaments directly associated with the joint capsule are generally in agreement with other descriptions for man and for ungulates, except that the fibres extend to include most of the margins of the fibrous as well as the synovial capsular parts of the joint.