Twenty-four hour plasma free insulin and blood glucose and intermediary metabolite profiles have been measured in 6 C-peptide deficient 'brittle' diabetic patients, during continuous sc and im insulin infusion. During sc infusion free insulin profiles were erratic and unpredictable. Mean 24 h blood glucose levels were raised at 12.6 +/- 2.1 (SE) mmol/l, and 3-hydroxybutyrate at 0.24 +/- 0.08 mmol/l. Blood lactate (1.88 +/- 0.18 mmol/l) and glycerol (0.084 +/- 0.007) were also elevated. Insulin (im) restored free insulin profiles to the normal pattern as found in 'stable' diabetics on sc infusion, with characteristic post-meal peaks (49 +/- 7, 103 +/- 35, and 95 +/- 34 mU/l) and stable night-time levels. Mean 24 h blood glucose was 6.7 +/- 1.1 mmol/l (P less than 0-.05 compared to sc infusion) and 3-hydroxybutyrate 0.07 +/- 0.02 mmol/l (P less than 0.05). Blood lactate (1.67 +/- 0.08 mmol/l) and glycerol (0.10 +/- 0.02 mmol/l) levels remained abnormal. The ratio of plasma free insulin to insulin dose administered was significantly higher during im infusion. In the 6 'stable' diabetics on sc insulin infusion good blood glucose control (7.1 +/- 0.9 mmol/l) was accompanied by clear post-prandial insulin peaks, and stable nocturnal levels. The results strongly suggest that in one category of 'brittle' diabetics there is defective and erratic sc insulin absorption.