This study evaluated in pregnant women with diabetes mellitus (DM) the prolactin (PRL) concentration in amniotic fluid (A-PRL), the initial decidual PRL content (D-PRL), and the decidual PRL secretion (D-PRL-s) in vitro into isosmotic (315 mmol/kg), hyperosmotic (426 mmol/kg) and hyposmotic (252 mmol/kg) media. Decidual tissue was collected at term from 18 normal pregnancies and from 23 women with DM (White's classification: 7 of class A (without insulin treatment) and 16 of the classes B-F. Twelve of the women with DM had signs of hydramnion. Amniotic fluid specimens were collected from 52 normal pregnancies and 17 of the women with DM. No significant difference was found between normals and diabetics when D-PRL, D-PRL-s into isosmotic medium and A-PRL were compared. No trends could be detected when the insulin treated women were grouped according to White's classification. The increment of D-PRL-s into hyperosmotic medium was 15% in normals (p less than 0.001 compared with isosmotic medium), in the DM group 23% for the non-insulin-treated women (p less than 0.05 when compared with normals) and 25% for the insulin treated women (p less than 0.01). In normals the hyposmotic medium reduced D-PRL-s by secretion 19% (p less than 0.001 compared with isosmotic medium) but no significant difference was observed between normals and diabetic groups (p greater than 0.10). In the group of diabetics no significant difference was found in any of the PRL quantities investigated between those with and without hydramnion.(ABSTRACT TRUNCATED AT 250 WORDS)