Apomorphine, a centrally-acting emetic, was administered subcutaneously (50 micrograms/kg) to nine normal subjects (four male, five female; aged 22-36 years) and four patients with idiopathic diabetes insipidus (DI) (one male, three female; aged 24-49 years). In the normal subjects this stimulus caused nausea (and vomiting in seven of nine) with a latency of 9.5 +/- 0.9 min which was followed by a large increase in plasma arginine vasopressin (AVP) concentration (from 0.9 +/- 0.2 pmol/l to 249 +/- 104 pmol/l at 15 min after the onset of symptoms; mean +/- SEM, P less than 0.01). There was a small but significant increase in plasma oxytocin (OXT) concentration (from 1.6 +/- 0.4 pmol/l to 6.2 +/- 3.4 pmol/l; P less than 0.05). Mean arterial pressure (MAP) fell slightly (from 87 +/- 1.9 mm Hg to 71 +/- 4.4 mm Hg; P less than 0.05) 15 min after the onset of nausea; there was no change in blood haematocrit or plasma osmolality and sodium concentration. In the DI patients apomorphine produced nausea (with vomiting in three of four) with a latency of 10.0 +/- 1.4 min but failed to cause an increase in either plasma AVP or OXT. In the DI patients the fall in MAP did not reach statistical significance (83 +/- 4 mm Hg to 71 +/- 11 mm Hg); there was also no change in haematocrit, osmolality or sodium concentration. Ipecacuanha, an emetic with both peripheral and central actions, was administered orally to seven normal subjects (three male, four female; aged 22-36 years) six of whom also underwent apomorphine tests.(ABSTRACT TRUNCATED AT 250 WORDS)