Metabolic and isotopic dilution techniques were used to investigate fluid balance and fluid volumes in rats made diabetic with streptozotocin before and after infusion. Uninfused diabetic rats had significantly (P < 0·01) lower total body water than controls (57·7±2·2 vs 65·7±1·4% (s.e.m.) fat free mass). This was due exclusively to a significantly (P < 0·001) reduced intracellular fluid volume (38·2±1·5 vs 45·4±1·4% fat free mass). Metabolic studies over the preceding 2 weeks showed that the fluid deficit in the diabetic group had resulted from a failure of the rats to increase their fluid intake to the same extent as their combined fluid losses. A 4-h saline infusion halved the fluid deficit in diabetic animals. The retained fluid was used to restore intracellular fluid volume which became comparable in diabetic and control rats (47·2±2·0 vs 46·4±1·0% fat free mass). The retention of infusate by diabetic animals to counteract their intracellular dehydration may partly explain the reduced urine output reported elsewhere in infused anaesthetized diabetic rats. Journal of Endocrinology (1991) 128, 333–337