Insulin therapy was withdrawn from 15 well-controlled type I diabetic subjects for no longer than 18 h to examine the sequence with which 13,14-dihydro-15-keto-PGE2 (PGE-m), glucagon, norepinephrine, and epinephrine increased in circulating blood in diabetic subjects becoming ketoacidotic. Fourteen of 15 patients had increments in PGE-m; 12/12, 12/15, and 13/15 had increments in glucagon, norepinephrine, and epinephrine, respectively. Six of the 15 patients developed mild diabetic ketoacidosis (DKA) by 12-18 h; all had nonmeasurable C-peptide levels. This DKA group had significantly greater increments of PGE-m (835 +/- 130 versus 276 +/- 111 pg/ml, mean +/- SEM, P less than 0.01) but not glucagon, norepinephrine, or epinephrine compared with the 9 non-DKA patients. In the DKA group, there were significant PGE-m and glucagon increments in the circulation by 3 h, significant norepinephrine increments by 9 h, and epinephrine increments in 5/6 patients by 12 h (not statistically significant) of insulin withdrawal. These studies document that (1) PGE-m accumulates in the circulation during DKA, (2) PGE-m and glucagon increase before catecholamines, and (3) PGE-m, glucagon, and catecholamine levels promptly return to normal levels when insulin therapy is reinstituted. It is suggested that elevated PGE-m levels early in the onset of DKA may represent a host-defense mechanism.