In 8 patients with diabetic ketoacidosis, serum prolactin was measured and found to be elevated (24.8 +/- 10.2 ng/ml, mean +/- SEM). After correction of the ketoacidosis, the prolactin level decreased to 10.9 +/- 6.4 ng/ml (normal range men 4.9 +/-0.8, women 5.1 +/- 1.6 ng/ml). The scatter of prolactin values was wide. A study of the correlation between the serum prolactin values and chemical parameters that are altered in diabetic ketoacidosis was therefore undertaken. Serum prolactin values did not correlate well with the serum bicarbonate concentration or serum osmolality. However, a significant negative correlation between log serum prolactin concentration and serum sodium concentration was demonstrated (r = -0.61, P less than 0.01). It is suggested that serum prolactin may possible participate in sodium retention in man as has been demonstrated in studies on animals.