1. Intakes and urine outputs of fluid and electrolytes were measured daily in rats before, and for 3 weeks after, induction of diabetes by intraperitoneal injection of streptozotocin (STZ; 60 mg/kg); control animals received saline.
2. Water intakes and urine outputs were increased on and after the first day after injection with STZ; after a transient period of negative water balance, fluid intakes and urine outputs increased in parallel.
3. Food intake was reduced for the first 3 days after injection of STZ but thereafter there was a steady increase. On the final experimental day, the food intake of the diabetic group was 60% greater than that of the control group.
4. Urinary electrolyte excretion was increased after injection of STZ; at the end of the experiment, the increase in urinary sodium excretion was similar to the increase in intake but the increase in urinary potassium excretion was less.
5. On day 21 after injection of STZ plasma sodium concentration and packed cell volume were significantly reduced in the diabetic group but plasma potassium concentration was not.
6. There was a difference between the measured osmolality and the calculated osmolarity of the plasma of the diabetic animals which was not seen in the controls. This difference was not due to pseudohyponatraemia, but was probably due to the presence of unidentified solutes, since there was a significant gap between the urinary osmolal and osmolar excretion in the diabetic animals that was not present in the control animals.