To evaluate pancreatic beta-cell secretory activity, c-peptide concentrations before and 6 minutes after an i.v. injection of glucagon were measured in 12 normal subjects (group N), 23 diabetics treated with diet control and oral antidiabetics (group DI) and 71 patients treated with insulin (group DII). The results showed that the fasting c-peptide concentration in group N and group DI were comparable, but significantly higher than that in group DII. An overlap was found between fasting c-peptide from 6 diabetics in group DI and that from group DII. There were significant differences among the c-peptide concentrations 6 minutes after glucagon injection in the three groups and there was no overlap between c-peptide in group DI and that in group DII 6 minutes after injection. The above-mentioned data suggest that c-peptide concentration 6 minutes after glucagon injection may reflect beta-cell secretory activity better than fasting c-peptide.