Skin fibroblasts were isolated from newborn spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) and cultured with 10% fetal calf serum. The growth rate, determined by the culture dish DNA content, was significantly higher in cells from SHR than from WKY in the presence of 10 and 15% serum. Re-initiation of DNA synthesis by serum and insulin was assessed by 24-h 3H-thymidine incorporation in quiescent cells. Serum (5, 10 and 15%) and insulin (1 microgram/ml) induced a marked stimulation of 3H-thymidine incorporation, and the cell response was significantly higher in SHR than in WKY for both agonists. These results indicate that the hyper-responsiveness to growth factors evident in newborn SHR skin fibroblasts is a genetic defect independent of haemodynamic changes.