The time course of lactacidemia was studied prospectively in 17 patients during fluid resuscitation for an episode of noncardiogenic shock, in 5 patients after grand mal seizures, and in 5 patients after successful CPR for cardiac arrest. The 9 patients in whom shock was reversed with fluid administration demonstrated a regular decrease in lactate concentrations, which exceeded 5% of the initial value during the first 60 min of treatment. In the other patients who expired despite similar therapy, lactacidemia was not significantly affected. During circulatory shock, repeated lactate determinations represent a more reliable prognostic index than an initial value taken alone. Changes in lactate concentration can provide an early and objective evaluation of the patient's response to therapy.