Twenty-four hour profiles of plasma C-peptide, an index of endogenous insulin secretion, were performed in 15 Type 1 (insulin-dependent) diabetic children. Plasma C-peptide was detectable in six children, of whom four ('C-peptide producers') had peak values above normal fasting levels. In each of the six children with residual B cell function, there was a close correlation between plasma C-peptide and simultaneous blood glucose (r greater than 0.50, p less than 0.05). Post-breakfast peak blood glucose was 10.2 +/- 1.7 mmol/l (mean +/- SEM) in the 'C-peptide producers' and 18.7 +/- 1.7 mmol/l in the 11 children with low or no detectable C-peptide. Mean M-value, an index of deviation from an ideal blood glucose, was lower in the 'C-peptide producers' (p less than 0.05). It is concluded that residual functioning B cells in diabetic children behave physiologically in that insulin secretion fluctuates in accordance with the prevailing blood glucose; and that the pattern of action of injected insulin is more critical in non-C peptide producers who lack the post-prandial dampening effect provided by residual endogenous insulin secretion.