This project was designed to examine the intracellular location of progesterone in bovine luteal cells. These experiments demonstrated the existence of a particulate membrane fraction of luteal cells where much of the endogenous progesterone was located. Results suggest an association between this fraction and the plasma membrane. Moreover, it was shown that these membranes were able to bind exogenous radiolabelled progesterone in a highly specific manner. Other steroids, precursors etc. were bound poorly. Thus, the experiments herein describe the characterisation of this novel progesterone binding site, its distribution in the cells of the bovine corpus luteum and preovulatory follicle, and attempts to purify and identify the progesterone binding protein. Results from these experiments indicated that the progesterone binding site investigated was distinct from classical genomic progesterone receptors. This non-classical progesterone binding protein (NCP4-BP) was found in both large and small luteal cells of the corpus luteum, though levels were greater in large cells. NCP4-BP was also found in the theca and granulosa cells of the preovulatory follicle. Binding characteristics of the NCP4-BP were determined, and partial purification achieved. Results demonstrate that progesterone binding was not due to (i) steroid metabolizing enzymes (ii) non-specific intercalation of steroid into bi-layer membranes or (iii) the genomic progesterone receptor. Studies suggest that the binding site studied may represent a membrane located progesterone receptor with a potential role in the regulation of luteal function in cows.