Vascular smooth muscle cell (SMC) growth is under the influence of various growth factors. We demonstrate that platelet-derived growth factor (PDGF) stimulates DNA synthesis of cultured bovine aortic SMCs by 2.5- to 3.5-fold. PDGF also exhibits additivity with insulin and insulin-like growth factor I (IGFI) for DNA synthesis and cellular proliferation. Insulin (2 × 10⁻⁶m), IGF-I (1 × 10⁻⁸m), and PDGF (1 × 10⁻⁹m) cause a 60–80% increase in cell numbers over basal, but PDGF with insulin or IGF causes a 40–150% increase over basal. No additivity between insulin and IGF-I is evident. PDGF also induces commitment to DNA synthesis earlier than insulin or IGFI. After exposure to PDGF for 4 h, SMCs incorporate ³H-thymidine to 60% of maximum (with PDGF alone) levels (achieved after exposure of 12 h or longer). Insulin and IGF-I exposure for 4 h, on the other hand, achieves ³H-thymidine incorporation that is only a 20–30% of maximum (with insulin or IGF-I alone). Insulin, IGF-I, and PDGF increase mRNA levels of the protooncogene c-myc. This induction begins within 30 min of exposure to these growth factors which causes a 4- to 6-fold increase in c-myc mRNA levels. Additivity is also observed between PDGF with insulin or IGF-I, but not between insulin or IGFI, in c-myc induction. C-myc mRNA levels remain elevated as long as the hormones are present, although there's a tendency for the mRNA levels to fall off with insulin and IGF-I. These data suggest that c-myc induction is an early marker for growth in response to these growth factors. Furthermore, these growth factors interact in the regulation of SMC growth, but do so via discrete pathways as evidenced by additivity in growth and c-myc induction. Characterization of insulin, IGF-I, and PDGFs growth effect on cultured vascular SMCs and on specific genes such as c-myc which may be associated with growth control, may provide an understanding of the abnormal growth of SMCs in atherosclerosis.