An acute oral protein load causes a transient hyperfiltration that might reveal a loss of glomerular permselectivity properties. The effect of an oral protein load on the urinary excretion rate of albumin (UAE), beta-2-microglobulin (UB2MGE), and retinol-binding protein (URBPE) was thus examined in cadmium- and lead-exposed workers. The results show a transient increase in UB2MGE and URBPE without relevant changes in UAE. These changes were also observed in control groups and are due to competitive inhibition of tubular protein reabsorption by absorbed cationic amino acids. These results do not support the hypothesis that the acute protein load test is of great utility in revealing a silent glomerular filtration disturbance in lead- or cadmium-exposed workers. However, it might disclose imminent renal tubular dysfunction in cadmium workers not yet showing increased microproteinuria under baseline conditions.