Small peptide molecules known as low molecular weight growth factor (LMW-GF) have been identified in human serum. They enhance the effect of IGF-1 (insulin-like growth factor) on proteoglycan synthesis. In the present work we investigated the role played by the kidney in the production of LMW-GF, using the pig as an experimental model. Six pigs underwent bilateral nephrectomy followed 24 h later by orthotopic autotransplantation of the kidney. Renal and liver functions were evaluated by measurement of serum creatinine, urea, electrolytes, amino transferases (ASAT, ALAT), proteins, and bilirubin. LMW-GF was measured by bioassay using 11-day-old pelvic chick embryo cartilages. We observed that LMW-GF quickly disappeared from pig serum after nephrectomy and only reappeared when transplantation was successful. Reappearance of LMW-GF can precede improvement of renal function evaluated by plasma creatinine levels. These data appear to demonstrate that the kidney is involved in LMW-GF production.