The action of IGF-I (insulin-like growth factor I) on the synthesis of tropoelastin in chick embryonic aortae was examined. Maximal and selective stimulation of the relative (40%) and absolute (145%) rates of tropoelastin synthesis over control occurred at an IGF-I concentration of 100 ng/ml of medium. Parallel to the increase in synthesis was a 92% increase in the amount of tropoelastin activity per 100 ng of poly (A)+ RNA translated in a cell-free system. The relative rate of tropoelastin synthesis achieved at maximal stimulation is greater than that observed during normal aortic embryogenesis. The stimulatory action of the hormone on elastin synthesis appears to be at a pretransitional level perhaps involving increased transcription or stabilization of the tropoelastin mRNAs. These results suggest that IGF-I may play a key role in the regulation of elastogenesis in arterial tissue.