The effect of acute water deprivation for 96 h on renal function was studied in male Sprague-Dawley rats. The glomerular filtration rate (GFR) and the effective renal plasma flow (ERPF) were estimated using inulin and p-amino hippuric acid (PAH), respectively, as model compounds. Acute water deprivation caused a significant decrease (44 per cent) in the GFR without altering the ERPF. These results indicate that a proper dosage adjustment may have to be made in the dehydration condition, especially for drugs whose disposition is dependent on GFR.