We have characterized the role of tyrosine phosphorylation in protooncogene induction mediated by insulin-like growth factors I and II (IGF-I and IGF-II) in the Madin-Darby canine kidney (MDCK) cell line. These cells possess few, if any, insulin receptors, thus allowing determination of the effects of these growth factors in the absence of any secondary signal mediated through the insulin receptor. We found that IGF-I produced a specific stimulation of tyrosine kinase activity of the 97-kDa beta-subunit of the IGF-I receptor, resulting in autophosphorylation of the receptor and an increase in kinase activity toward a synthetic peptide substrate. This was associated with a gradual decrease in the level of phosphorylation of pp120, the major constitutive phosphotyrosine-containing protein of MDCK cells, and an increase in the ratio of serine to tyrosine phosphorylation. This was followed by a rapid, but transient, induction of c-fos gene expression, with no change in the levels of c-myc mRNA. Cycloheximide treatment resulted in a superinduction of both c-fos and c-myc and prevented any further stimulation by IGF-I. IGF-II did not stimulate tyrosine phosphorylation of its own receptor, but was 25% as active as IGF-I in stimulating phosphorylation of the IGF-I receptor. Despite this, IGF-II did not significantly enhance the expression of either nuclear protooncogene. Insulin also produced a delayed stimulation of IGF-I receptor phosphorylation, but was unable to stimulate biological effects in these cells. Under these conditions neither of the IGFs nor insulin produced any significant stimulation of thymidine incorporation into DNA. These data indicate that the IGF-I receptor can be activated upon binding of IGF-I, and to a lesser extent IGF-II, in intact cells to mediate cellular events. The nature of the signal generated by the IGF-I receptor appears to vary depending on the ligand that occupies it.