Serum concentrations of ALA and PBG have been measured in normal subjects, patients with AIP, and lead workers. Both porphyrin precursors are significantly increased in serum from porphyric patients, and serum ALA is elevated in lead workers. Endogenous clearance measurements, when compared with creatinine clearances, are consistent with significant tubular reabsorption of ALA and PBG under physiologic circumstances, and with an "overflow" mechanism for increased excretion in AIP or lead intoxication. Three of 8 porphyric patients showed an aminoaciduria; tyrosine was involved in 2, tryptophan in the other 3. Evidence to suggest a specific renal defect contributing to increased ALA excretion in chronic lead intoxication was not obtained.