The effects of the ovarian steroids, oestradiol benzoate (EB), and progesterone (P) on the cells of pars intermedia (PI) from chronically ovariectomized rats (CHR-OVX) were analyzed by qualitative and quantitative electron microscopy (EM) at different intervals after steroid injection. The PI cells of CHR-OVX are rich in secretory granules but poor in organelles related to hormonal synthesis. Twenty-four h after EB administration the cells exhibited cytological features indicative of an increased synthetic activity. These included hypertrophy of rough endoplasmic reticulum, cisternae, a moderately developed Golgi complex, and newly formed granules. These features were also observed in PI cells 48 h after EB administration. Thirty-two and 56 h after the treatment, the PI cells showed signs of both increased synthetic and secretory activity. Thus, it was possible to observe a well-developed rough endoplasmic reticulum, and Golgi apparatus, numerous electron-lucent vesicles, and secretory granules in contact with the cell membrane. However, no exocytotic figures were observed. Progesterone administration resulted in considerable modifications of the ultrastructural features of PI cells also indicative of increased synthetic and secretory activity. The greatest modifications were observed in the mornings with changes that were 12 h out of phase with respect to those observed with EB. Quantitative estimations of the variation in the content of secretory granules of PI cells fully confirmed the qualitative observations described above. The serum alpha-MSH concentrations in ovariectomized rats was found to be increased 24 h after administration of a single dose of EB and thereafter serum MSH exhibited high levels in the afternoon, whereas the values in the morning were lower.