Morphological changes in human granulosa cells in culture were observed by phase, fluorescent, scanning electron and transmission electron microscopy following the addition of human chorionic gonadotropin (hCG), luteinizing hormone (LH), 8-bromocyclic adenosine 3',5'-monophosphate (cAMP) and cytochalasins B and D. In response to these agents, polygon-shaped granulosa cells with granular cytoplasm became rounded, leaving fingerlike processes attached to the substratum and adjacent cells. The changes in cell shape were accompanied by a centripetal movement of mitochondria and lysosomes to a perinuclear location. The morphological alterations appeared to be mediated by cyclic AMP and to be the result of a dismantling and reorganization of microfilament-containing stress fibers. Follicle-stimulating hormone (FSH), prolactin (PRL), growth hormone (GH), and human placental lactogen (hPL) did not provoke cell shape changes. We conclude that tropic hormones capable of stimulating progestin secretion by luteinized granulosa cells cause a change in cell structure in vitro which leads to a redistribution of organelles involved in steroid synthesis. The possible relationship of the cytoskeleton to steroidogenesis is considered.