Qualitative and quantitative changes of tertiary follicles were studied in ovaries of heifers during and after the treatment with melengestrol acetate (MGA--6-methyl-6-dehydro-16-methylen-17alpha-acetoxyprogesteron), the atretic tertiary follicles being classified into several types. During the administration of MGA an increased number of atretic follicles was found as well as a stimulation of glandular cells of theca interna, while after the withdrawal of the drug an increased number of normal tertiary follicles was observed estrous cycle. On the 7th day of MGA administration most of the atretic follicles were in the stage of early atresia and a few only were in the stage of luteinized cystic atresia. On the 14th day, however, most of tertiary follicles were in late atresia and only a few were in early atresia. Moreover, at the same time the number of follicles classified as luteinized cystic atresia was increased, while that of follicles showing cystic follicular atresia was decreased. After the withdrawal of MGA the most numerous group of follicles was in the stage of early atresia. It was demonstrated that the time changes in the number of follicles with a diameter larger than 5 mm resemble that of normal tertiary follicles. During MGA administration the number of follicles with a diameter of 2 to 5 mm was the largest. These findings show that MGA may effect the feed-back regulation of both the cyclic and basal secretion of gonadotropic hormones. After the withdrawal of the drug there may be higher basal secretion of gonadotropins.