In children with chronic renal insufficiency serum levels of somatomedin measured by radioreceptor assay were found to be strikingly elevated and were in the same range as in acromegaly in spite of decreased growth. The serum somatomedin level was inversely correlated with renal function and children on haemodialysis had the highest values. The elevated somatomedin was most likely due to progressive destruction of the kidney, the primary catabolic site for somatomedin and other polypeptides. After successful transplantation the somatomedin values fell to slightly above normal even though growth was still impaired. Using a bioassay based on the mitogenic property of somatomedin, a lower than normal rather than an increased level was found in chronic renal insufficiency suggesting that in uraemia an inhibitor to somatomedin bioactivity was present. It is concluded that the cause of the growth failure in chronic renal insufficiency and after transplantation is not due to a lack of somatomedin, but an inhibitor to its action could be a factor. It would appear that a normal somatomedin may be necessary for normal growth, but it is not sufficient.