In labile diabetes mellitus wihout ketoacidosis we have studied plasma prolactin levels and a possible causal connection between fluctuation in blood glucose concentration and plasma prolactin, growth hormone and cortisol levels. The hormone concentrations in plasma and blood glucose concentration were determined at 20 min intervals for a 24 h period in six male patients with insulin treated diabetes mellitus. Prolactin varied within the normal range but without any significant rise in relation to sleep in five out of the six patients. Growth hormone levels were low with superimposed secretory peaks. Plasma cortisol showed a normal diurnal rhythm. Blood glucose fluctuated as expected, but the variations and especially the falls in blood glucose to non-hypoglycaemic levels were not followed by increases in plasma hormone concentrations. No relationship could be demonstrated between the changes in the plasma concentration of prolactin, growth hormone and cortisol.