To better characterize the effects of body position and exercise on urinary protein excretion, carefully defined random urine samples were obtained during recumbency and following both ambulation and exercise in healthy adolescent student athletes. Albumin, lysozyme, and N-acetyl-B-D-glucosaminidase were measured in all samples. Glomerular permeability and tubular function were assessed using the urinary albumin creatinine ratio (UAlb/UCr), the urinary lysozyme creatinine ratio (ULy/Ucr), the urinary N-acetyl-B-D-glucosaminidase creatinine ratio (UNa/UCr), and the urinary lysozyme albumin ratio (ULy/ UAlb)- UAlb/Ucr was significantly (p<0.001) lower in recumbent urine samples than in either ambulatory or post exercise samples, although no difference was seen between the latter two groups. Furthermore, recumbent UAlb/UCr was higher in females (/ 0.01) and post exercise UAlb/UCr varied significantly (p<0.001), depending on the type of physical activity. ULy/UCr, UNag/UCr, and ULy/UAlb were unaffected by either posture or physical activity. A significant correlation was found between UAlb/UCr(r=0.60, p=0.0001) and also between UNag/UCr and ULy/UAib (r=0.84, p=0.001). In addition, urine specific gravity was found to have a significant negative correlation with UAlb/Ucr (r=-0.33, p=0.001). The results of this study suggest that in the adolescent, recumbent albumin excretion is higher in females and that ambulation increases glomerular permeability. Exercise does not appear to induce any additional alteration in glomerular permeability, although the effects of exercise are likely related to the type and severity of physical activity. Renal tubular function is unaltered by either ambulation or exercise. Furthermore, urinary albumin excretion may be increased in the face of diuresis and urinary N-acetyl-B-D-glucosaminidase excretion may be stimulated by the effects of increasing albumin excretion. (Pediatr Res 21: 442-446, 1987)