The ovaries of six multiparous cows 6-8 years old were collected 49-71 h after injection of a synthetic prostaglandin F analogue (cloprostenol) and serially sectioned. All follicles greater than 0.4 mm in diameter were counted and measured. The number of normal follicles ranged from 46 to 360 per cow. Atresia was not seen in follicles of less than 1.7 mm diameter. The number of early atretic follicles ranged from 4 to 20 per cow. The volume and mitotic index of the granulosa tissue was calculated for all follicles. Follicles were then grouped into classes according to granulosa volume, each class having twice the volume of granulosa of the preceding class. In all cows the rate of growth of follicles was slow up to 0.5 mm diameter (class 4). Above this size growth accelerated to a maximum in follicles of 1.2-1.7 mm diameter (class 8) and declined thereafter to a minimum in follicles 8-10 mm in diameter (class 13). Follicles beyond class 13 were not present in any of the ovaries. It was estimated that 22.1 days were required for a follicle to grow from 0.4 mm to 10 mm in diameter (classes 4 through to 13). The present data suggest that the pattern of synchronization of oestrus in cattle treated with cloprostenol may be related to the size of the largest non-atretic follicle present at the time of treatment because of the time required for such a follicle to complete its development.