1. The effect of age and sex on glomerular filtration rate (GFR) was measured by the 51Cr-EDTA radioisotopic method in 76 normal individuals (43 women and 33 men). 2. Age has a significant effect on GFR. Subjects aged 41 to 60 years have GFR values [104.5 +/- 16.5 ml min-1 (1.73 m2)-1, N = 43] lower than younger individuals aged 20 to 40 years [116.6 +/- 11.2 ml min-1 (1.73 m2)-1, N = 33]. GFR decreases after 40 years of age by approximately 6.0 ml min-1 (1.73 m2)-1 per decade. 3. GFR values in women [105.9 +/- 16.0 ml min-1 (1.73 m2)-1, N = 43] were lower when compared to men [114.8 +/- 14.3 ml min-1 (1.73 m2)-1, N = 33]. 4. We conclude that the effect of sex and age must be taken into account when establishing reference values for GFR.