The parenchyma of the corpus luteum of the ewe consists of two distinct steroidogenic cell types: small luteal cells and large luteal cells. Although both cell types produce and secrete progesterone, they differ with respect to morphological and biochemical characteristics. During the oestrous cycle, and continuing into pregnancy, the cellular composition of the corpus luteum is altered. As the oestrous cycle progresses small luteal cells increase in number but not size whereas large luteal cells remain constant in number but increase in size. Changes in the cellular composition of the ovine corpus luteum appear to be regulated by LH. Moreover, small luteal cells obtained from pregnant ewes were larger and lacked responsiveness to LH compared to those from non-pregnant animals. The basis of this loss of responsiveness is not clear as there is no concomitant loss of receptors for LH. The corpus luteum is a dynamic gland which changes in cellular composition and hormonal responsiveness with alterations in the reproductive state of the animal.