Addition of ng/ml quantities of the platelet-derived growth factor (PDGF) component of serum to the culture medium of quiescent monolayers of BALB/c-3T3 cells rapidly altered the actomyosin cytoskeletal system. PDGF triggered dispersal of microfilament bundles and induced phosphorylation of the 20 kD myosin light chain within 5 min of addition. Additional cytoskeletal responses followed the addition of insulin and of epidermal growth factor (EGF). Insulin alone induces a transient breakdown of well defined microtubule filaments. EGF and insulin together induced moderate perturbation of the cytoplasmic network of intermediate filaments which then reformed within a 90-180 min time frame. This response, however, was greatly enhanced in cells which were pretreated with PDGF and it appears to be a secondary event occurring as a consequence of cell growth. The PDGF-induced disruption of microfilament bundles could be blocked by phalloidin with no apparent inhibition of the mitogenic response. Insulin alone is only weakly mitogenic for 3T3 cells. The data thus indicate that the architecture of two discrete cytoskeletal elements (microtubules and microfilaments) is regulated directly by separate serum growth factors (insulin and PDGF). However the disruption of microfilament architecture by PDGF does not seem necessary for induction of cell growth and the disruption of microtubule filaments by insulin is not sufficient.