Selenium (Se) is a trace element variously distributed in the human body and especially concentrated in certain organs, such as the renal cortex. We report results obtained during a ten weeks' oral Se supplementation. Experiments were devised to evaluate previous preliminary observations which suggested a possible effect of Se addition on the renal glomerular filtration rate. Eleven healthy volunteers have given increasing oral Se (as a sodium selenite solution) as follows: on the first week they have given 100 micrograms Se per day; this was progressively increased 100 micrograms per day for each of the following 6 weeks; the last dose (700 micrograms per day) was maintained for three further weeks. Serum and 24-hour urine were collected weekly for creatinine determination by kinetic Jaffé reaction and Se measurement by proton-induced X ray emission (PIXE). The final mean serum creatinine concentration was 13% lower than the initial mean value (p less than 0.01). Mean creatinine clearance increased significantly (p less than 0.05) and showed a direct correlation with mean Se clearance (r = 0.79; p less than 0.001). As the increase of creatinine clearance was concomitant with a reduction of serum creatinine levels, we excluded the possibility of toxic effects. Our results seem to suggest a positive influence of Se supplementation on the rate of glomerular filtration and we hypothesize that Se might be involved in the vascular regulatory mechanism of the kidney.