Urine production rate is determined by the amount of the antidiuretic hormone (ADH), which increases the water permeability of collecting duct epithelial cells. The higher the plasma concentration of ADH, the higher the amount of water that is reabsorbed. At maximal ADH levels, a very small volume of highly concentrated urine is produced. In the absence of ADH, a very large volume of very dilute urine is produced. See also this graph on the Relationship between plasma antidiuretic hormone (ADH) concentration and urine osmolality and urine production rate.