The insulin secretory capacity of three groups of Nigerian African diabetics was assessed by measuring the concentration of C-peptide before and after stimulation with oral glucose. Group 1 subjects had a history of keto-acidosis and were treated with insulin. Those in group 2 had no history of keto-acidosis but required insulin to normalize blood glucose while those in group 3 also had no history of keto-acidosis and were treated with diet alone or in combination with oral hypoglycaemic drugs. C-peptide levels (mean +/- s.e.m.) showed group 1 subjects to be insulin deficient (fasting 0.08 +/- 0.04 pmol/ml, peak 0.14 +/- 0.03 pmol/ml), group 2 to have reduced insulin secretion (fasting 0.16 +/- 0.01 pmol/ml, peak 0.35 +/- 0.01 pmol/ml) and group 3 to have a moderately reduced fasting insulin and a higher peak insulin secretion (fasting 0.27 +/- 0.03 pmol/ml, peak 1.49 +/- 0.4 pmol/ml) compared with a non-diabetic control group (fasting 0.30 +/- 0.03 pmol/ml, peak 1.16 +/- 0.1 pmol/ml). Although the aetiology of diabetes in the Nigerian African is unclear, a spectrum of the disease exists which is similar to that in Caucasian population.