We studied 10 patients with congestive heart failure to assess the dynamics of their response to 40 mg of furosemide. Patients excreted less sodium than normal controls: 142 +/- 36 and 245 +/- 16 mEq/4 hr (p < 0.05). Patients delivered the same amount of furosemide into the urine--14.9 +/- 2.0 and 18.7 +/- 2.1 mg/4 hr (p < 0.20)--but the time course of delivery differed. Normal subjects had a sigmoid-shaped curve when furosemide excretion rate was related to response. All patients but one had shifts in this curve and for a number of patients the configuration differed substantially from a sigmoid curve.