Renal biopsies of 44 patients with endocapillary acute glomerulonephritis (gn) and 64 patients with moderately severe mesangioproliferative gn were investigated morphometrically (point-counting-method, tubulometry). In both gn's statistically significant positive corrleations between relative interstitial volume and the concentration of serum creatinine at the time of biopsy were found. Despite severe glomerular lesions the serum creatinine concentration is not increased in most cases of endocapillary acute gn, providing the relative interstitial volume is not increased by more than 15%. Increased serum creatinine concentration without a markedly enlared interstitium was found in 11 cases of endocapillary acute gn with clinically and morphologically proven acute renal failure. In these cases the glomerular function is probably impaired by the Thurau-mechanism. In all other patients, especially in those with moderately severe mesangioproliferative gn, the serum creatinine concentration rises with an enlargement of relative interstitial volume. This reduction of renal function may be explained by a decrease to the total cross-sectional area of postglomerular vessesl, caused by interstitial fibrosis. That may possible lead to diminished renal blood flow and glomerular filtration with an increase of the serum creatinine concentration.