The involvement of microtubules in the gonadotropic regulation of granulosa cell steroidogenesis was assessed at the preantral (E2-cells) and antral (PMS-cells) stages of follicular development. The influence of agents that alter microtubule-tubulin equilibrium on basal and FSH-stimulated progesterone production was determined in vitro and compared with that on microtubule integrity and organization using immunofluorescence. Basal and FSH-stimulated progesterone production was approximately 2-fold higher in PMS-cells than in E2 cells. Colchicine and nocodazole, two agents that depolymerize microtubules, significantly stimulated progesterone and 20 alpha-hydroxypregn-4-en-3-one production in PMS-cells. Although progesterone production by E2-cells was increased by nocodazole, the amount produced was considerably less than that produced by PMS-cells. FSH-stimulated progesterone biosynthesis was reduced by colchicine and nocodazole in both cell types. Taxol, an agent that stabilizes microtubules, markedly reduced FSH-stimulated progesterone production in both E2- and PMS-cells, but failed to exert a comparable effect on basal steroid production. A close association existed between the concentrations of colchicine, nocodazole, and taxol that altered basal and/or FSH-stimulated steroidogenesis and those that affected microtubule organization and/or distribution. Whereas granulosa cells appeared flattened with numerous cytoplasmic processes after 24 h of culture in medium alone, they were almost spherical and devoid of projections after culture with these agents. FSH-stimulated cells also occupied less area than controls, although cytoplasmic processes were present. These findings indicate an involvement of microtubules in the regulation of granulosa cell steroidogenesis. It is proposed that one of their roles is to facilitate the movement of cholesterol from lipid droplets to mitochondria, possibly by bringing these cellular inclusions closer together.