The subcellular distribution and compartmentalization of choriomammotrophin (CM) and progesterone within ovine placentomes was investigated using differential and density gradient centrifugation techniques. Approximately 67% of placental CM and 45% of progesterone was associated with subcellular particles. The 10 000 g particulate fraction contained the highest specific activity of both CM and progesterone (19·1 ±3·8 (s.e.m.) μg/mg and 71·5 ± 9·2 pmol/mg protein respectively). This fraction was also shown to contain electron-dense granules with morphology similar to that of hormone-containing secretory granules isolated from other endocrine tissues. Particle-associated CM sedimented to a density of 1·051–1·054 g/ml in colloidal silica gradients and displayed physicochemical characteristics consistent with its storage in secretory granules. During in-vitro incubations, particle-associated CM was stable for up to 90 min, but dissociated when incubated in hypoosmotic medium. Particulate progesterone, which was also present in the CM-rich fraction and was stable for up to 90 min of incubation, was not affected by decreasing the osmolality of the incubation medium. These data suggest that ovine CM (but not progesterone) is stored within a population of secretory granules located within placentomes.