The addition of ACTH to primary cultures of functional mouse adrenocortical tumor cells results in a marked increase in the production and release of steroids by these cells. The steroidogenic response to ACTH is detectable within minutes and is associated with a 'rounding-up' of the cells. The effects of ACTH were analyzed by scanning, conventional transmission and stereoscopic high voltage electron microscopy. Cell morphology was studied in unstimulated cells, in cells stimulated for 10-15 min in which only partial rounding had occurred, and after 2 hr of continuous stimulation when most of the cells had fully rounded. In unstimulated cultures, bundles of microfilaments (stress fibers) were prominent in cell extensions, subjacent to coated regions of the plasma membrane and frequently in proximity to microtubules and clusters of lysosome-like organelles. As the rounding process commenced, stress fibers disappeared from the cell edges. In 2 hr stimulated cells, Golgi complexes were hypertrophied. There were more microprojections from the plasma membrane than in control cells, as well as evidence of increased pinocytotic activity. There was an apparent polymerization of microtubules in elongated processes extending outward from the bases of the rounded cells and a concentration of lysosome-like organelles in these formations. The possible significance of these changes with respect to the differentiated function of adrenal cells is under continuing investigation in our laboratory.