The ultrastructure of normal ovarian hilar cells, both during the reproductive era and after the menopause, shows evidence of steroid hormone synthesis. In this respect these cells are very similar to Leydig cells in that they contain mitochondria with a dense matrix, a highly developed smooth endoplasmic reticulum, Reinke crystals, and microcystalline inclusions. The latter are much more numerous than in the normal testis. Their enzymatic activities are quite similar to those of Leydig cells. Ovarian hilar cells seem to originate from progressive metamorphosis of fibroblastic cells under the induction of sympathetic nervous structures. Their relations with hilar nerves are of three types: simple direct membrane contact without a Schwann cell sheath, intimate intracytoplasmic relationships, and specialized contacts, which are comparable to a synapse.