The effects of phalloidin and cytochalasin D, drugs which, respectively, stabilize and destabilize actin microfilaments, have been tested on isolated rat hepatocytes. Both drugs produced a modification of cell shape, characterized by protrusions bulging from the cytoplasm. In phalloidin-treated hepatocytes, an accumulation of actin microfilamentous network was detectable at the base of each protrusion by electron microscopy, immunofluorescence, and HMM decoration. This accumulation of microfilaments was absent in cytochalasin D-treated cells. The release of triglycerides, an index of very low density lipoprotein secretion, was inhibited by phalloidin or cytochalasin D, and accompanied by an increase in cellular triglycerides. At the electron microscope examination, triglyceride accumulation was represented by fat droplets and vesicle-enclosed, very low density lipoprotein-like particles. Total protein and albumin secretion was only very slightly modified by either one of these drugs. With the use of various phalloidin analogs, a correlation was observed between their respective ability to stabilize F-actin in vitro, and their effects on cell shape and triglyceride secretion. In conclusion, phalloidin, and cytochalasin D: (a) modify the shape of isolated hepatocytes; (b) inhibit lipoprotein secretion. These effects possibly result from a modification of actin microfilament function.