In the vole, Microtus agrestis, there was no significant change in the number of corpora lutea (cl) during either pregnancy or pseudopregnancy. In pregnant animals, two waves of luteal development took place and histological changes during the two phases of growth differed. In pseudopregnant animals, a similar increase occurred initially which was followed by a marked decrease after Day 8, at which time cl began to degenerate. Luteinized follicles were visible early in pregnancy and pseudopregnancy. Graafian follicles were present after Day 2 or 3. In pregnant animals, the number remained more or less constant, except for a slight increase on Day 19, but their maximum size was reduced at about mid-term, whereas, in pseudopregnant animals, a marked increase in size occurred between Days 8 and 10. Vaginal smears and histology indicated progesterone secretion up to Day 19 of pregnancy and for the 8 to 9 days of pseudopregnancy. There was, therefore, a good correlation between the presence of healthy cl and smears that indicated progesterone secretion.W. G. Breed and J. R. Clark