C-Peptide is secreted from the beta-cell in equimolar quantities with insulin. Since a fraction of C-peptide is excreted in the urine, measurement of C-peptide in timed urine collections is a simple indirect measure of integrated insulin production. Normal subjects were studied to determine the effects of diet and oral prednisone on urinary C-peptide excretion. In subjects on a defined diet, there is a positive correlation of urinary C-peptide with body weight. When insulin production is increased after oral prednisone, there is also a positive correlation with body mass index and percent ideal body weight. Prednisone increases plasma glucose, immunoreactive insulin, and serum and urinary C-peptide levels beginning 8-12 h after oral administration. This effect of prednisone is most marked in the postprandial state. Diets high in carbohydrate and protein result in significantly more insulin production, as measured by urinary C-peptide, than isocaloric diets with low protein or carbohydrate composition.