Renal function and osmoregulation were studied in bobwhite quail (Colinus virginianus) raised with unrestricted water (chronically unrestricted group) or restricted water (chronically restricted group). There was no difference in urine concentrating ability between adult and juvenile (3.5 or 7.5 week-old) quail. A filtration marker (polyethylene glycol) was infused into adult quail via osmotic minipumps and responses to the following regimens studied: ad libitum water intake, short-term (4day) water restriction, and acute (1-day) dehydration (withdrawal of all drinking water). Chronically restricted quail had higher urine-to-plasma ratios of polyethylene glycol and lower urine flow rates during short-term restriction. A greater proportion of the reduction in urine flow rate during dehydration was attributable to enhanced tubular reabsorption, rather than reduced rates of filtration, in chronically restricted than in chronically unrestricted birds. Chronically restricted birds also had higher maximum urine-to-plasma ratios of polyethylene glycol (but not higher urine osmolality). These differences occurred in the face of arginine vasotocin concentrations that were not different in the two groups of birds (approximately 15 pg.ml 1 during hydration, and 45 pg.ml 1 during water restriction or dehydration). These observations suggest that chronically restricted quail have an enhanced responsiveness of tubular reabsorption to dehydration, a finding consistent with previous observations of tubule hypertrophy and hyperplasia in these birds (Goldstein and Ellis 1991). Despite this, no difference was found in medullary cAMP levels, either basal or Abbreviations: ADH, antidiuretic hormone; AVT, arginine vasotocin; rob, body mass; cAMP, cyclic adenosine-monophosphate; DEH, birds raised with restricted water intake; dpm, decays per minute; ECF, extracellular fluid; ECFV, extracellular fluid volume; Epic, total rate of polyethylene glycol excretion; GFR, glomerular filtration rate; Hct, hematocrit; HYD, birds raised with unrestricted water intake; PEG, polyethylene glycol; P .... plasma osmolality; Pp~, plasma concentration of polyethylene glycol; UeEc, urine concentration of polyethylene glycol; (U/P)eEo, urine-to-plasma ratio concentration of polyethylene glycol; V, urine flow rate arginine vastotocin-or forskolin-stimulated, in the two groups. When given water ad libitum, chronically restricted quail drank copiously (more than two times the drinking rate of chronically unrestricted birds rehydrating from acute dehydration or short-term water restriction), but glomerular filtration rate, hematocrit, and plasma osmolality did not differ in the two groups under this condition; chronically restricted quail excreted the excess water consumed during rehydration in a copious urine accomplished by reduced tubular water reabsorption.