We used a human renin antiserum and an immunoperoxidase method to investigate the distribution of renin-containing cells in 19 adult polycystic kidneys: 9 autopsy and 10 nephrectomy cases. These cells were present in residual normal kidney, in scarred renal parenchyma and in area of fibrous tissue. They were situated mainly in juxtaglomerular apparatuses (JGAs) and in the walls of arteries and arterioles. A semi-quantitative analysis showed hyperplasia of JGA renin-containing cells in the untreated autopsy cases. In both groups there was an abnormal distribution of renin-containing cells; only 50% were located in the JGAs the remainder were mainly in the walls of small arteries. These were often thin, attenuated vessels in the walls of cysts. Cells containing renin were also found isolated in fibrous tissue separate from the arterial tree. This abnormal location of the renin-containing cells suggests that they may respond to different stimuli than those in normal kidneys, and their abnormal distribution could affect both the intrarenal action of renin and also its release into the blood.