Exposure of bovine luteal tissue to the anti-microfilament agent cytochalasin B inhibited both basal and LH-induced steroidogenesis, but had no effect on the portion of the progesterone secreted into the incubation medium. Addition of 10% calf serum to the incubation medium caused no significant increase in progesterone synthesis, but produced a significant increase in the portion of the progesterone found in the incubation medium. This increased secretion in the serum-containing medium was not altered by the presence of LH, and cytochalasin B was unable to prevent it. Exposure of the luteal tissue to the antimicrotubular drugs, colchicine and vinblastine, produced no alteration in synthesis or secretion of progesterone, even when LH was present, and neither drug inhibited the serum-induced increase in secretion of progesterone. These studies suggest that luteal cell microfilaments may be involved in LH-stimulated progesterone production while microtubules apparently play no direct role in progesterone synthesis.