Previous studies of diabetic patients indicate that increased urinary excretion of certain plasma proteins (molecular radii <55 A), such as IgG, transferrin, and ceruloplasmin, precede the development of microalbuminuria. Moreover, increases in these urinary proteins predict future development of microalbuminuria. To clarify whether blood pressure changes influence urinary excretion of these proteins, we examined relationships between diurnal blood pressure changes measured by ambulatory blood pressure monitoring and urinary excretion of IgG, transferrin, ceruloplasmin, alpha2-macroglobulin (88 A) and albumin (36 A) measured separately during the day and night in 20 healthy controls and 26 normotensive, normoalbuminuric diabetic patients. Diurnal change in systolic blood pressure was not correlated to urinary excretion of either albumin or alpha2-macroglobulin in either diabetic patients or controls. However, statistically significant correlations between diurnal changes in systolic blood pressure and those of urinary excretion of IgG, transferrin and ceruloplasmin were found in diabetic patients but not in controls. The present findings suggest that urinary excretion of IgG, transferrin, and ceruloplasmin are more easily affected than albuminuria by systemic blood pressure changes in normoalbuminuric diabetic patients. This is supported by our previous finding that urinary excretion of IgG, transferrin and ceruloplasmin increased while albuminuria did not following enhanced glomerular filtration rate after acute protein loading, which causes increased glomerular capillary pressure due to afferent arterioles dilation, mimicking diabetic intra-renal hemodynamics. Taken together, these findings suggest that urinary excretion of IgG, transferrin, and ceruloplasmin may be more sensitive indicators of glomerular capillary pressure change than albuminuria in normoalbuminuric diabetic patients.