Exocrine pancreatic function was evaluated in patients with primary diabetes mellitus by oral administration of N-benzoyl-L-tyrosyl-p-aminobenzoic acid (pancreatic function diagnostic test, PFD test) and p-aminobenzoic acid (PABA absorption test). In both primary diabetes mellitus and chronic pancreatitis, the mean excretion of PABA in the urine in the PFD test was significantly less than in the controls, and in 19 of 31 (61.3%) patients with primary diabetes mellitus and 11 of 12 (91.7%) patients with chronic pancreatitis there was a low PABA excretion rate. In contrast, the mean excretion of PABA in the urine in the PABA absorption test was significantly less in those with primary diabetes mellitus than in the controls. Therefore, to detect disturbances of pancreatic exocrine function in patients with primary diabetes mellitus, differences in the excretion of PABA in the urine between PFD test and PABA absorption test should be calculated. According to this method, the rate of abnormality was 12.9% in primary diabetes mellitus and 100% in chronic pancreatitis. There was a significant correlation between the excretion of PABA in the urine in the PFD test and results of renal function tests in primary diabetic patients with a normal range of serum creatinine levels. The serum PABA levels in the PFD test remained high in patients with primary diabetes mellitus and decreased in cases of chronic pancreatitis, as compared with the controls.