The mean number of follicles per-field, follicular diameter, the follicular cell number and height, nuclear diameter and the volume of colloid were determined and statistically analysed in the thyroid gland of thirty six ewes during the different periods of estrus cycle and pregnancy.
The secretory activity of the throid gland was higher during the follicular phase of estrus cycle as well as the late stages of pregnancy (90-150 days) comparing to those of the luteal phase of the cycle and the early stages of pregnancy. However, the minimal activity was recognized during anestrus. The parafo llicular cells were more frequently encountered during the late stages of pregnancy.