In patients with chronic renal insufficiency undergoing hemodialysis, the seric zinc level was significantly lower than in normal subjects. In 7 patients this was even more marked prior to their first hemodialysis session. Following hemodialysis this level increased, but by comparing the zinc level to that of the binding proteins it could be seen that this result was a reflection of the hemo-concentration phenomenon. During hemodialysis there was no change in the zinc level of dialysate. Finally, a significant increase in the seric level of the 7 new patients was noted after 6 months of hemodialysis treatment. This increase reached the mean level observed in the 26 other patients who had been undergoing hemodialysis from between 8 and 87 months, but pre-dialysis normalization of seric zinc levels was not attained. The reasons for this increase in the 7 recently hemodialysed patients receiving no zinc treatment are discussed.